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Short Term Effects of Revegetation on Labile Carbon and Available Nutrients of Sodic Soils in Northeast China

Short Term Effects of Revegetation on Labile Carbon and Available Nutrients of Sodic Soils in... land Article Short Term E ects of Revegetation on Labile Carbon and Available Nutrients of Sodic Soils in Northeast China 1 , 2 , 3 1 , 3 1 , 2 , 3 2 , 4 Pujia Yu , Xuguang Tang , Shiwei Liu , Wenxin Liu * and Aichun Zhang Chongqing Jinfo Mountain Field Scientific Observation and Research Station for Karst Ecosystem, School of Geographical Sciences, Southwest University, Chongqing 400715, China; yupujia@swu.edu.cn (P.Y.); xgtang@swu.edu.cn (X.T.); liushiwei@swu.edu.cn (S.L.) Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, China State Cultivation Base of Eco-agriculture for Southwest Mountainous Land, Southwest University, Chongqing 400715, China College of Mobile Telecommunications, Chongqing University of Posts and Telecom, Chongqing 401520, China; zac12357@163.com * Correspondence: liuwx@iga.ac.cn; Tel.: +86-1874-301-9395 Received: 26 November 2019; Accepted: 30 December 2019; Published: 2 January 2020 Abstract: In response to land degradation and the decline of farmers’ income, some low quality croplands were converted to forage or grassland in Northeast China. However, it is unclear how such land use conversions influence soil nutrients. The primary objective of this study was to investigate the influences of short term conversion of cropland to alfalfa forage, monoculture Leymus chinensis grassland, monoculture Leymus chinensis grassland for hay, and successional regrowth grassland on the labile carbon and available nutrients of saline sodic soils in northeastern China. Soil labile oxidizable carbon and three soil available nutrients (available nitrogen, available phosphorus, and available potassium) were determined at the 0–50 cm depth in the five land uses. Results showed that the treatments of alfalfa forage, monoculture grassland, monoculture grassland for hay, and successional regrowth grassland increased the soil labile oxidizable carbon contents (by 32%, 28%, 15%, and 32%, respectively) and decreased the available nitrogen contents (by 15%, 19%, 34%, and 27%, respectively) in the 0–50 cm depth compared with cropland, while the di erences in the contents of available phosphorus and available potassium were less pronounced. No significant di erences in stratification ratios of soil labile carbon and available nutrients, the geometric means of soil labile carbon and available nutrients, and the sum scores of soil labile carbon and available nutrients were observed among the five land use treatments except the stratification ratio of 0–10/20–30 cm for available phosphorus and the values of the sum scores of soil labile carbon and available nutrients in the 0–10 cm depth. These findings suggest that short term conversions of cropland to revegetation have limited influences on the soil labile carbon and available nutrients of sodic soils in northeastern China. Keywords: revegetation; Solonetz; stratification ratio; geometric mean; Songnen plain 1. Introduction The structure, diversity, and production capacity of terrestrial ecosystems are strongly linked to the availability of soil nutrients, such as nitrogen, phosphorus, potassium, and soil organic carbon [1,2]. However, the soil labile carbon and soil nutrients’ availability in terrestrial ecosystems are usually influenced by various direct and indirect soil disturbances [3,4]. Land use conversions are major drivers of changes in soil labile carbon and soil nutrient availability, resulting in the degradation of soil ecosystem services (nutrient cycle, water conservation, pollution purification, etc.) and global Land 2020, 9, 10; doi:10.3390/land9010010 www.mdpi.com/journal/land Land 2020, 9, 10 2 of 14 environmental problems (soil degradation, climate change, water, soil erosion, etc.) [5,6]. One main mechanism by which they do this is by changing the quantity and quality of plant biomass supplied to the soils, a ecting the rate of organic matter decomposition and the activity of soil microorganism and redistributing soil carbon and nutrients within soil profiles [7–9]. Another main mechanism is the impact of soil erosion, which preferentially removes the surficial and most carbon and nutrient rich material, thus accelerating the decline of the soil organic carbon and soil nutrient pool [10,11]. Characterizing the spatial and temporal variability of soil carbon and nutrients in relation to land use types is critical for predicting the influences of future land use conversion on soil quality changes and understanding how ecosystems’ function [12]. Changes of soil organic carbon (soil microbial biomass carbon, particulate organic carbon, water extractable organic carbon, etc.) and total nutrients (total nitrogen, total phosphorus, total potassium, etc.) under di erent land uses in di erent spatial and temporal scales induced by long term land use conversions have been well addressed [13–17]. However, the evaluation of the short term e ects of land uses on soil labile carbon and soil available nutrients are rare due to the high spatial variability and the errors in the measurement methods of these soil properties [18,19]. Moreover, due to regional di erences in environment conditions, initial soil properties, and management years and intensity, inconsistent and contradictory responses of soil labile carbon and soil available nutrients to short term land use conversions were observed, which failed to demonstrate a clear relationship between short term land uses and changes in soil labile carbon and available nutrients [14,20]. For instance, Madejon et al. [14] in the southwest of Spain found that after three year plantation of fast growing trees decreased the contents of available nitrogen (AN), available phosphorus (AP), and available potassium (AK). Lu et al. [21] in Tibet, China, reported that short term (nine years) grazing exclusion had no impact on soil AN, AP, microbial biomass carbon, and other soil properties. However, the results of Wang et al. [20] in Shanxi Province, China, showed that three years of plantation of grass and alfalfa significantly increased the contents of soil organic carbon, AN, AP, and AK. As one of the largest salt a ected soil regions in China, Songnen plain has su ered from substantial land salinization and alkalization because of the influences of human activity in recent decades [22]. The increases in soil alkalinity and sodicity have adversely influenced the soil properties by promoting crusting and low permeability and infiltration rates [23], thus leading to the reduction of grain production. Furthermore, with an increase in corn production in China from 1.06 10 tons in 2003 to 2.25  10 tons in 2016 due to the increase of yield per unit and the planting area, oversupply resulted, causing a notable reduction in corn price and planting benefit to farmers [22]. Therefore, the croplands with poor quality soils were abandoned in the Songnen plain. To address these problems in the Songnen plain, the Chinese government implemented a range of policies and subsidies to guide farmers to improve the eciency and sustainable development of agriculture through revegetation in the areas where the soils were not suitable for growing crops. Revegetation, the conversion of cropland to a vegetation covered land, has become a well explored approach to rehabilitate degraded soil ecosystems [15,24]. In addition to combatting erosion and protect soils, revegetation has substantial e ects on the accumulation of soil organic carbon and nutrients and the improvement of soil microbial biomass and activity [13,15]. Deng et al. [15] in the Loess Plateau, China, found that the contents of soil organic carbon, total N, and total P at a depth of 0–20 cm increased by more than 13%, 10% and 11%, respectively, after 30 years of grassland restoration. Our previous study in the same site showed that soil microbial biomass carbon and enzyme activity at a depth of 0–20 cm increased by 36% and 56%, respectively, after five years of conversion of cropland to grassland [22]. However, the short term influences of revegetation on soil oxidizable carbon and available nutrients have yet to be quantified in northeastern China. In this study, we hypothesized that five years of revegetation from cropland could increase the contents of soil available nutrients and soil labile carbon and therefore be beneficial to the sustainable use of saline sodic soils in northeastern China. To address this hypothesis, the objective of this research was to investigate the changes in the contents of labile oxidizable carbon (LOC), AN, AP, and AK Land 2019, 8, x FOR PEER REVIEW 3 of 14 research was to investigate the changes in the contents of labile oxidizable carbon (LOC), AN, AP, Land 2020, 9, 10 3 of 14 and AK after conversion from cropland to alfalfa forage, monoculture Leymus chinensis grassland, monoculture Leymus chinensis grassland for hay, and successional regrowth grassland and examine after conversion from cropland to alfalfa forage, monoculture Leymus chinensis grassland, monoculture whether short term revegetation could improve the LOC and soil available nutrients in the regions Leymus chinensis grassland for hay, and successional regrowth grassland and examine whether short where the soils were not suitable for planting crops in northeastern China. term revegetation could improve the LOC and soil available nutrients in the regions where the soils were not suitable for planting crops in northeastern China. 2. Materials and Methods 2. Materials and Methods 2.1. Study Area 2.1. Study Area The research was conducted in the Songnen plain located at the Grassland Farming and Ecological Research Station (123°31′ E, 44°33′ N) (Figure 1). The terrain surrounding the study area is The research was conducted in the Songnen plain located at the Grassland Farming and Ecological relatively flat, and the altitude is approximately 145 m above sea level. The study area has a 0  0 Research Station (123 31 E, 44 33 N) (Figure 1). The terrain surrounding the study area is relatively temperate, semiarid continental climate. The average annual temperature is 5.9 °C, and the mean flat, and the altitude is approximately 145 m above sea level. The study area has a temperate, semiarid annual precipitation is 427 mm (1980–2013). The soil is classified as Solonetz in the World Reference continental climate. The average annual temperature is 5.9 C, and the mean annual precipitation is Base for Soil Resources with a soil texture of 22% sand, 33% silt, and 45% clay [25]. The main 427 mm (1980–2013). The soil is classified as Solonetz in the World Reference Base for Soil Resources vegetation consists of perennial herbs such as Leymus chinensis and Puccinellia tenuiflora. Besides, some with a soil texture of 22% sand, 33% silt, and 45% clay [25]. The main vegetation consists of perennial therophytes such as Chloris virgata and Suaeda heteroptera grow in the areas with higher soil pH and herbs such as Leymus chinensis and Puccinellia tenuiflora. Besides, some therophytes such as Chloris virgata poor soil quality [26]. and Suaeda heteroptera grow in the areas with higher soil pH and poor soil quality [26]. Figure 1. The location map of the study area. Figure 1. The location map of the study area. 2.2. Experimental Design 2.2. Experimental Design This experiment was organized as a completed block design with five land use treatments. In early This experiment was organized as a completed block design with five land use treatments. In May 2011, four adjacent blocks (each 60 50 m, 2 m bu er between the blocks) in the study area based early May 2011, four adjacent blocks (each 60 × 50 m, 2 m buffer between the blocks) in the study area on similar land use history were identified. Before this experiment (2004–2010), farmers grew rain fed based on similar land use history were identified. Before this experiment (2004–2010), farmers grew maize (Zea mays L.) and sunflower (Helianthus annuus) in these blocks, following the traditional planting rain fed maize (Zea mays L.) and sunflower (Helianthus annuus) in these blocks, following the practices in Northeast China, which consists of plowing the soil down to 20 cm depth and applying traditional planting pr 1actices in Northe 1ast China, which con1sists of plowing the soil down to 20 cm 50–96 kg N ha , 20–45 kg P ha , and 15–45 kg K ha fertilizers into the soils. The soil properties in −1 −1 −1 depth and applying 50–96 kg N ha , 20–45 kg P ha , and 15–45 kg K ha fertilizers into the soils. The these four blocks were homogeneous due to the continuous plowing. The five land use treatments soil properties in these four blocks were homogeneous due to the continuous plowing. The five land consisted of corn cropland (corn, used as an indication of how the revegetation influences the soils in use treatments consisted of corn cropland (corn, used as an indication of how the revegetation this study), alfalfa perennial forage land (alfalfa), monoculture grassland of Leymus chinensis (MLG), influences the soils in this study), alfalfa perennial forage land (alfalfa), monoculture grassland of monoculture grassland of Leymus chinensis for hay (Mowing) once a year (MLG + M), and successional Leymus chinensis (MLG), monoculture grassland of Leymus chinensis for hay (Mowing) once a year regrowth grassland (SRG) (Figure 2). Leymus chinensis is the native vegetation and is usually used as (MLG + M), and successional regrowth grassland (SRG) (Figure 2). Leymus chinensis is the native forage grass for grazing animals in the Songnen grassland. In the mowing grassland, Leymus chinensis vegetation and is usually used as forage grass for grazing animals in the Songnen grassland. In the is harvested as hay, and farmers sell the hay to livestock farms. Alfalfa has high saline and alkaline mowing grassland, Leymus chinensis is harvested as hay, and farmers sell the hay to livestock farms. tolerance, and it has been introduced into the Songnen grassland as a high forage plant due to the Alfalfa has high saline and alkaline tolerance, and it has been introduced into the Songnen grassland high N and protein content [22]. The planting of forage grass used in this study could improve the as a high forage plant due to the high N and protein content [22]. The planting of forage grass used income of local farmers and the development of animal husbandry. In each block, two greater plots of in this study could improve the income of local farmers and the development of animal husbandry. 12 50 m were for corn and alfalfa treatments, while three plots of 6 50 m for land use treatments of MLG, MLG + M, and SRG. There was a 1 m bu er among the five plots. The was no irrigation under Land 2019, 8, x FOR PEER REVIEW 4 of 14 Land 2019, 8, x FOR PEER REVIEW 4 of 14 Land 2020, 9, 10 4 of 14 In each block, two greater plots of 12 × 50 m were for corn and alfalfa treatments, while three plots of In each block, two greater plots of 12 × 50 m were for corn and alfalfa treatments, while three plots of 6 × 50 m for land use treatments of MLG, MLG + M, and SRG. There was a 1 m buffer among the five 6 × 50 m for land use treatments of MLG, MLG + M, and SRG. There was a 1 m buffer among the five the five land uses in this study. More information about the treatments of land uses is presented in plots. The was no irrigation under the five land uses in this study. More information about the plots. The was no irrigation under the five land uses in this study. More information about the Figure 3 [22,26]. treatments of land uses is presented in Figure 3 [22,26]. treatments of land uses is presented in Figure 3 [22,26]. Figure 2. Images of the land use treatments in this study. Corn, corn cropland; Alfalfa, alfalfa forage Figure 2. Images of the land use treatments in this study. Corn, corn cropland; Alfalfa, alfalfa forage Figure 2. Images of the land use treatments in this study. Corn, corn cropland; Alfalfa, alfalfa land; MLG, monoculture grassland; MLG + M, monoculture grassland for hay (Mowing); SRG, land; MLG, monoculture grassland; MLG + M, monoculture grassland for hay (Mowing); SRG, forage land; MLG, monoculture grassland; MLG + M, monoculture grassland for hay (Mowing); SRG, successional regrowth grassland. successional successionalr re egr gr owth owth grassland. grassland. Successional Monoculture Monoculture Alfalfa perennial forage Corn cropland (Corn) Successional Monoculture Monoculture Alfalfa perennial forage Corn cropland (Corn) regrowth grassland grassland of Leymus grassland of Leymus (Alfalfa) regrowth grassland grassland of Leymus grassland of Leymus (Alfalfa) (SRG) chinensis for hay chinensis (MLG) Since 2011, the cropland (SRG) chinensis for hay chinensis (MLG) Since 2011, the cropland (MLG + M) Before 2014, the Alfalfa has been under continuous (MLG + M) Before 2014, the Alfalfa has been under continuous Seeds of Leymus plots were no tillage corn monoculture. The The cropland was Seeds of Leymus plots were no tillage corn monoculture. The The cropland was chinensis (Trin.) cropland, and other Corn plots followed the abandoned in 2011 in Seeds of Leymus chinensis (Trin.) cropland, and other Corn plots followed the abandoned in 2011 in Seeds of Leymus Tzvelev were sowed practices were the same as traditional cropland the SRG plots to chinensis (Trin.) Tzvelev were sowed practices were the same as traditional cropland the SRG plots to chinensis (Trin.) in May 2011 with a those of the previously practice in the Songnen restore grassland Tzvelev were sowed in May 2011 with a those of the previously practice in the Songnen restore grassland Tzvelev were sowed density of described corn cropland. grassland; this tradition without any in May 2011 with a density of described corn cropland. grassland; this tradition without any in May 2011 with a approximately 2000 However, the growth of consists of plowing the disturbance. The density of approximately 2000 However, the growth of consists of plowing the disturbance. The density of −2 seeds m . Reseeding corn was very poor in 2011 soil at least twice before dominant species in approximately 2000 −2 seeds m . Reseeding corn was very poor in 2011 soil at least twice before dominant species in approxim 2 ately 2000 had a positive effect to 2013 due to the poor soil the crop growing season the SRG plots include seeds/m . Reseeding had a positive effect to 2013 due to the poor soil the crop growing season the SRG plots include seeds/m . Reseeding on the recovery of conditions and short term down to 20 cm and Chloris virgate, had a positive effect in −1 on the recovery of conditions and short term down to 20 cm and Chloris virgate, had a positive effect in vegetation, and the land use, and the no tillage fertilization (74 kg N ha , Sonchus brachyotus, recovering the −1 −1 vegetation, and the land use, and the no tillage fertilization (74 kg N ha , Sonchus brachyotus, recovering the aboveground biomass cropland was changed to 22 kg P ha , and 41 kg K Chenopodium vegetation. The −1 −1 aboveground biomass cropland was changed to 22 kg P ha , and 41 kg K Chenopodium vegetation. The reached alfalfa forage land in May ha ) twice per year at glaucum, etc. The aboveground biomass −1 reached alfalfa forage land in May ha ) twice per year at glaucum, etc. The aboveground biomass approximately 100– 2014 with a sowing density sowing and in mid-July. aboveground (348 g was mowed for hay −2 −2 approximately 100– 2014 with a sowing density sowing and in mid-July. aboveground (348 g was mowed for hay 120 g m in early of approximately 1200 The corn straw was m ) and belowground once a year at the peak −2 −2 −2 −2 120 g m in early of approximately 1200 The corn straw was m ) and belowground once a year at the peak September 2011. The seeds m . The removed from the plots (397 g m , 0–20 cm biomass. The −2 −2 −2 September 2011. The seeds m . The removed from the plots (397 g m , 0–20 cm biomass. The aboveground (381 g aboveground (307 g m ) after harvest, while the depth) biomass was belowground (638 g −2 −2 −2 aboveground (381 g aboveground (307 g m ) after harvest, while the depth) biomass was belowground (638 g m ) and belowground and belowground (321 g corn root, stem base, and kept as litter to return m , 0–20 cm depth) −2 −2 −2 −2 m ) and belowground and belowground (321 g corn root, stem base, and (456 g m , 0–20 cm m , 0–20 cm depth) aerial root (which is 137 g kto ep the so t as litte il. r to return m biom , ass 0–2 0 was cmk ep detp th to) −2 −2 −2 d (45 epth 6 ) g b m iom , ass 0– 20 was cm bio mm, ass we 0–2re 0 kep cm t in th dese epth ) m aeria ) we l ro re ot (which incorpo irated s 137 g to the soil. b retu iom rn ass to was the so kep ils t in to −2 kept as litter to return plots to increase soil into soil during plowing. depth) biomass was biomass were kept in these m ) were incorporated retu these p rn lo tots. th e soils in to the soil. fertility in 2014 and 2015. kept as litter to return plots to increase soil into soil during plowing. these plots. to the soil. fertility in 2014 and 2015. Figure 3. Detailed information and history of land use treatments in this study. Figure 3. Detailed information and history of land use treatments in this study. Figure 3. Detailed information and history of land use treatments in this study. 2.3. Soil Sampling and Analysis 2.3. Soil Sampling and Analysis 2.3. Soil Sampling and Analysis Soil sampling was performed using an auger (4 cm in diameter) in early September 2015. Soil sampling was performed using an auger (4 cm in diameter) in early September 2015. The The sampling depth was 0–50 cm with an interval of 10 cm increments. Five randomly distributed Soil sampling was performed using an auger (4 cm in diameter) in early September 2015. The sampling depth was 0–50 cm with an interval of 10 cm increments. Five randomly distributed sub- sub-samples from each plot were combined into a composite sample at each soil depth. After removing sa sa mp mp ling les fr dep omth ea was ch plot 0–50 we cre m com with bined an interv into al a com of 10 poc sm ite incremen sample at tsea . F ch ive soril ando depmly th. After distrrem ibuted oving sub- the visible vegetation materials and debris, soil samples were sieved through a 2 mm sieve, and then sa th mp e visi les bfr le om vegetation each plot m ate werials re com and bined debris int , soi o a l com samp po les site were samp sieve le d at th ea rough ch soil a dep 2 mm th . siAfter eve, an rem d th ov en ing ground to pass through a 0.25 mm sieve for analyses. ground to pass through a 0.25 mm sieve for analyses. the visible vegetation materials and debris, soil samples were sieved through a 2 mm sieve, and then Soil labile oxidizable carbon (LOC) was measured using the revised method defined by ground Soil to pa labile ss oxi throu dizabl gh a 0.25 e carbon mm (LOC) sieve f wor as a meas nalyse ured s. using the revised method defined by Chan et Chan et al. [27]. Available nitrogen (AN) was measured by the alkaline hydrolysis di usion method [28]. al. [27]. Available nitrogen (AN) was measured by the alkaline hydrolysis diffusion method [28]. The Soil labile oxidizable carbon (LOC) was measured using the revised method defined by Chan et The AN AN forms forms were wer prim e primarily arily mixtmixtur ures of es am of mo ammonium nium nitrogen nitrogen (NH3-(NH N), nitr -N), ate nitrate nitrogen nitr (NO ogen 3-N) (NO , and -N), a al. [27]. Available nitrogen (AN) was measured by the alkaline hydrolysis 3 diffusion method [28] 3. The and small a small amoun amount t of water of water soluble soluble organic organic nitrogen nitr(e ogen .g., amino (e.g., amino acids and acids am and moniu ammonium m acyl, etc acyl, .). AN forms were primarily mixtures of ammonium nitrogen (NH3-N), nitrate nitrogen (NO3-N), and a Available phosphorus (AP) was extracted with NaHCO3 at pH 8.5 and measured using UV etc.). Available phosphorus (AP) was extracted with NaHCO at pH 8.5 and measured using UV small amount of water soluble organic nitrogen (e.g., amino 3 acids and ammonium acyl, etc.). spectrophotometer [28]. The AP forms were primarily the calcium phosphates due to the higher soil spectrophotometer [28]. The AP forms were primarily the calcium phosphates due to the higher soil Available phosphorus (AP) was extracted with NaHCO3 at pH 8.5 and measured using UV pH (Table 1) in the study area. Available potassium (AK) was measured based on the ammonium pH (Table 1) in the study area. Available potassium (AK) was measured based on the ammonium spectrophotometer [28]. The AP forms were primarily the calcium phosphates due to the higher soil acetate extracted and emission flame spectrophotometer method [28]. The AK forms were primarily pH (Table 1) in the study area. Available potassium (AK) was measured based on the ammonium Land 2020, 9, 10 5 of 14 mixtures of exchangeable potassium and water soluble potassium. Furthermore, for the purpose of clearly understanding the nature of soils in the study area, a 1:5 soil:water solution was used to measure the soil pH and electrical conductivity (EC) using the PHS-3C instrument and the DDS-307 instrument, respectively. Table 1. Mean values (SE) of soil pH and electrical conductivity (EC) under di erent land uses. Soil Corn Alfalfa MLG + M MLG SRG ANOVA Depth F P pH 0–10 9.36 (0.14) 9.14 (0.04) 9.16 (0.10) 9.00 (0.10) 9.05 (0.26) 0.88 0.50 10–20 9.83 (0.17) 9.76 (0.09) 9.93 (0.09) 9.87 (0.06) 9.39 (0.43) 1.01 0.43 20–30 10.06 (0.08) 10.04 (0.09) 10.03 (0.04) 10.00 (0.06) 9.96 (0.08) 0.29 0.88 30–40 10.11 (0.04) 10.11 (0.08) 10.03 (0.03) 9.99 (0.04) 10.02 (0.03) 1.11 0.39 40–50 10.06 (0.06) 10.04 (0.09) 9.99 (0.04) 9.92 (0.04) 9.98 (0.05) 0.78 0.56 EC 0–10 204 (16) 155 (11) 162 (5) 157 (21) 164 (24) 1.43 0.27 10–20 330 (56) 306 (43) 398 (53) 376 (32) 335 (101) 0.36 0.84 20–30 391 (49) 385 (65) 531 (55) 491 (39) 435 (95) 1.00 0.44 30–40 389 (31) 391 (58) 576 (46) 501 (55) 492 (75) 2.12 0.13 40–50 350 (17) 349 (59) 481 (56) 468 (64) 452 (45) 1.63 0.22 Abbreviations: Corn, corn cropland; Alfalfa, alfalfa forage land; MLG, monoculture grassland; MLG + M, monoculture grassland for hay; SGR, successional growth grassland. 2.4. Statistical Analysis The soil stratification by certain properties (e.g., SOC, total N, total P, etc.) is very common, and the stratification ratio (SR) is widely used as a crucial indicator of soil condition [15]. A higher SR of soil properties indicates better soil conditions, because SR of degraded soils is usually less than 2 regardless of climatic or soil conditions [15]. The improvement of soil quality under specific land use is conducive to plant growth and agricultural sustainability [29,30]. Revegetation on the cropland will increase the input of organic matter and thus alter the SR of soil properties, which will provide an indication of soil responses to specific plant cover. The SR were calculated for each land use as follows: SR = ANt/ANs (1) where AN is the content of LOC, AN, AP, and AK in the 0–10 cm depth; AN is the corresponding t s content of LOC, AN, AP, and AK in the 10–20 and 20–30 cm depth. A unitary soil available nutrient is not complete to reveal the changes within the soil environment because soil available nutrients do not always respond similarly to di erent management practices [22]. Therefore, the comprehensive assessment of the responses of a series of soil available nutrients and LOC to factors of change is required. However, the various responses of LOC and soil available nutrients to land use change might result in inaccurate conclusions on soil quality and thus limit the suitability of LOC and soil available nutrients as soil quality indicators. The geometric mean and sum scores are two general indices to combine the variables with diverse units and ranges into one variable, which could clearly indicate the actual influences of environmental factor changes on these variables [31]. Here, the geometric means of LOC and available nutrients (GMSN) under di erent land uses and soil depths are calculated as follows: 1/4 GMSN = (LOC AN AP AK) (2) where LOC, AN, AP, and AK are oxidizable labile C, available nitrogen, available phosphorus, and available potassium, respectively. Land 2020, 9, 10 6 of 14 The simple sum of the series of soil available nutrients and LOC with di erent units and ranges of variation may cover up the changes in some soil nutrients. Therefore, data normalization is needed for all the measured soil available nutrients and LOC before the sum scores of LOC and available nutrients (SSAN) are calculated. The min-normalization is a well explored approach to convert the data with di erent units or variation ranges into a dimensionless pure value, so that the data can remove the unit limit and can be easily compared and weighted [26]. The SSAN under di erent land uses and soil depths is as follows: S = X/X (3) i min SSAN = S (4) i=1 where S is the score of LOC, AN, AP, and AK after data normalization; X is the measured value, and X is the minimum value of each soil nutrient observed in this study; n is the number of soil nutrients. min We used one way ANOVA to analyze the influences of land use types on the LOC and soil available nutrients, soil pH, EC, SR, GMSN, and SSAN. Mean di erences of soil available nutrient contents, LOC, SR, GMSN, and SSAN among land use treatments were examined using the least significant di erence test (LSD). All comparisons were considered significant if p < 0.05. The mean and standard error of each soil property measured were provided at each soil depth under a given land use treatment. All data analyses were performed with SPSS 16.0 for Windows (SPSS, Inc., Chicago, USA). 3. Results 3.1. Changes in Soil pH and EC Soil pH in the study area was notably high (Table 1). The values of soil pH were all more than 9.00 at the 0–50 cm depth; especially at the 10–50 cm depths, the values were close to or more than 10.00. Soil pH was not a ected by the land use conversions. The average values of soil pH at the 0–50 cm depth were 9.88, 9.82, 9.83, 9.76, and 9.68 for corn, alfalfa, MLG, MLG + M, and SRG treatment, respectively. Similar to the soil pH, the EC values in the subsoil (10–50 cm) were higher than that at the surface soil (0–10 cm). The average values of EC in the 0–50 cm depth were 333, 317, 430, 399, and 376 S -1 cm for corn, alfalfa, MLG, MLG + M, and SRG treatment, respectively (Table 1). There was no significant di erence of EC among the land use treatments because of the narrow values of EC in the same soil depth. 3.2. Changes in LOC, AN, AP, and AK Content The LOC content under the land use of SRG was remarkably higher than that under corn in the 0–10 cm depth, while it was significantly higher under alfalfa in the 10–20 cm depth than the corn and MLG + M treatment (Figure 4A). In the 20–50 cm depth, the highest LOC content was found under the MLG treatment. The average LOC contents in the 0–50 cm depth were 32% (0.56 g kg ), 28% 1 1 1 (0.49 g kg ), 15% (0.26 g kg ), and 32% (0.57 g kg ) higher under alfalfa, MLG, MLG + M, and SRG treatment, respectively, than that under corn treatment. Land use conversions significantly (F = 8.76, p = 0.001) changed the AN contents (Figure 4B). The highest AN content was found under corn treatment in the 0–50 cm depth. In addition, the AN contents under corn treatment in the 20–30 cm and 40–50 cm depth were significantly higher than those under all the revegetation land except the alfalfa treatment in the 20–30 cm depth. The average 1 1 1 AN contents in the 0–50 cm depth were 15% (6.3 mg kg ), 19% (8.0 mg kg ), 34% (14.9 mg kg ), and 27% (11.8 mg kg ) lower under alfalfa, MLG, MLG + M, and SRG treatment, respectively, than under corn treatment. Land 2020, 9, 10 7 of 14 Land 2019, 8, x FOR PEER REVIEW 7 of 14 -1 LOC (g kg ) 0 1 2 3 4 5 6 7 8 ab 10 ab ab 20 ab ab NS ab Corn 40 a ab Alfalfa MLG MLG + M ab SRG ab -1 AN (mg kg ) 0 20 40 60 80 100 ab ab ab ab 20 ab ab Corn 40 NS Alfalfa MLG MLG + M SRG 50 bc bc Figure 4. Mean values of labile oxidizable carbon (LOC) (A) and available nitrogen (AN) (B) under Figure 4. Mean values of labile oxidizable carbon (LOC) (A) and available nitrogen (AN) (B) under di erent land uses. The bars represent standard errors. Values without a common letter within land different land uses. The bars represent standard errors. Values without a common letter within land use treatments di ered according to the LSD test (p < 0.05). NS = not significant among di erent land use treatments differed according to the LSD test (p < 0.05). NS = not significant among different land uses. See Figure 2 for the abbreviations. uses. See Figure 2 for the abbreviations. The di erences of AP and AK contents among the soil depths in the 0–50 cm depth were very The differences of AP and AK contents among the soil depths in the 0–50 cm depth were very narrow except the 0–10 cm depth (Figure 5). Compared with the land uses of MLG, MLG + M, and SRG, narrow except the 0–10 cm depth (Figure 5). Compared with the land uses of MLG, MLG + M, and land use of corn had a higher AP content at the 0–10 cm depth. However, land use treatments did not SRG, land use of corn had a higher AP content at the 0–10 cm depth. However, land use treatments change the AP contents at the 10–50 cm depth (Figure 5A). The average AP contents in the 0–50 cm did not change the AP contents at the 10–50 cm depth (Figure 5A). The average AP contents in the 0– depth under the land uses of corn, alfalfa, MLG, MLG + M, and SRG were 4.1, 3.8, 3.4, 3.1, and 3.4 mg 50 cm depth under the land uses of corn, alfalfa, MLG, MLG + M, and SRG were 4.1, 3.8, 3.4, 3.1, and −1 3.4 mg kg , respectively. The highest AK contents were all found under the MLG treatment in the 0– Soil depth (cm) Soil depth (cm) Land 2020, 9, 10 8 of 14 Land 2019, 8, x FOR PEER REVIEW 8 of 14 kg , respectively. The highest AK contents were all found under the MLG treatment in the 0–50 cm 50 depth cm d (Figur epth e (Fi 5B). gure However 5B). Howev , significant er, sign di ifi cer ant ences differe wer nces e only wer found e onlbetween y found bet MLG ween andMLG corn tr an eatment d corn in the 20–40 cm depth and between MLG and alfalfa treatment in the 30–40 cm depth. The average AK treatment in the 20–40 cm depth and between MLG and alfalfa treatment in the 30–40 cm depth. The aver contents age AK in the cont 0–50 ents cm in th depth e 0–50 under cm dep corn, th under alfalfa, corn MLG, , alfalfa MLG , M +L M, G, and MLG SRG + Mtr , eatment and SRGwer treatmen e 102.9, t −1 112.2, 137.5, 108.7, and 125.8 mg kg , respectively. were 102.9, 112.2, 137.5, 108.7, and 125.8 mg kg , respectively. -1 AP (mg kg ) 0 2 4 6 8 10 12 Corn Alfalfa MLG MLG + M SRG 0-10 cm: MLG, MLG + M, SRG < Corn Other soil depths: NS -1 AK (mg kg ) 0 50 100 150 200 250 300 350 400 Corn Alfalfa MLG MLG + M SRG 20-30 cm: Corn < MLG 30-40 cm: Corn, Alfalfa < MLG Other soil depths: NS Figure 5. Mean values of AP (A) and AK (B) under di erent land uses. AP represents available Figure 5. Mean values of AP (A) and AK (B) under different land uses. AP represents available phosphorus. AK represents available potassium. The bars represent standard errors. NS = not phosphorus. AK represents available potassium. The bars represent standard errors. NS = not significant among di erent land uses. significant among different land uses. 3.3. Changes in SR, GMSN, and SSAN The SR of LOC, AN, and AK in the 0–10/10–20 cm and in the 0–10/20–30 cm (Figure 6) were not affected by the different land uses. Land uses of corn and alfalfa had remarkably higher SR values of Soil depth (cm) Soil depth (cm) Land 2020, 9, 10 9 of 14 3.3. Changes in SR, GMSN, and SSAN Land 2019, 8, x FOR PEER REVIEW 9 of 14 The SR of LOC, AN, and AK in the 0–10/10–20 cm and in the 0–10/20–30 cm (Figure 6) were not a ected by the di erent land uses. Land uses of corn and alfalfa had remarkably higher SR values of AP AP in the 0–10/20–30 cm depth than the land uses of MLG, MLG + M, and SRG. However, the in the 0–10/20–30 cm depth than the land uses of MLG, MLG + M, and SRG. However, the di erences differences of the SR value for AP in the 0–10/10–20 cm depth were not significant among the five of the SR value for AP in the 0–10/10–20 cm depth were not significant among the five treatments. treatments. All the SR values of LOC, AN, AP, and AK in the 0–10/10–20 cm depth were all <2 except All the SR values of LOC, AN, AP, and AK in the 0–10/10–20 cm depth were all <2 except the values the values of AN under SRG treatment, AP under corn and alfalfa treatment, and AK under MLG of AN under SRG treatment, AP under corn and alfalfa treatment, and AK under MLG treatment. treatment. However, the SR values of LOC, AN, AP, and AK in the 0–10/20–30 cm depth were all >2, However, the SR values of LOC, AN, AP, and AK in the 0–10/20–30 cm depth were all >2, except the except the values of AN under corn treatment and AP under MLG and MLG + M treatment. values of AN under corn treatment and AP under MLG and MLG + M treatment. Figure 6. Changes in the stratification ratio (SR) of 0–10/10–20 cm (A) and 0–10/20–30 cm (B) under Figure 6. Changes in the stratification ratio (SR) of 0–10/10–20 cm (A) and 0–10/20–30 cm (B) under di erent land uses. The bars represent standard errors. Values without a common letter within land use different land uses. The bars represent standard errors. Values without a common letter within land treatments di ered according to the LSD test (p < 0.05). NS = not significant among di erent land uses. use treatments differed according to the LSD test (p < 0.05). NS = not significant among different land uses. Values of GMSN in the 0–50 cm depth and SSAN in the 10–50 cm depth were not influenced by the changes of land use (Figure 7). The highest GMSN value was found under alfalfa in 0–20 cm depth, Values of GMSN in the 0–50 cm depth and SSAN in the 10–50 cm depth were not influenced by while it was highest under MLG treatment in the 20–50 cm depth (Figure 7A). The average GMSN the changes of land use (Figure 7). The highest GMSN value was found under alfalfa in 0–20 cm values in the 0–50 cm depth under corn, alfalfa, MLG, MLG + M, and SRG treatment were 13.1, 13.2, depth, while it was highest under MLG treatment in the 20–50 cm depth (Figure 7A). The average 13.3, 11.2, and 12.7, respectively. The SSAN values under corn and alfalfa treatment in the 0–10 cm GMSN values in the 0–50 cm depth under corn, alfalfa, MLG, MLG + M, and SRG treatment were 13.1, 13.2, 13.3, 11.2, and 12.7, respectively. The SSAN values under corn and alfalfa treatment in the 0–10 cm depth were markedly greater than the land use of MLG + M (Figure 7B). The average SSAN values in the 0–50 cm depth under corn, alfalfa, MLG, MLG + M, and SRG treatment were 9.5, 9.0, 9.7, 8.6, and 9.1, respectively. Land 2020, 9, 10 10 of 14 depth were markedly greater than the land use of MLG + M (Figure 7B). The average SSAN values in the 0–50 cm depth under corn, alfalfa, MLG, MLG + M, and SRG treatment were 9.5, 9.0, 9.7, 8.6, and 9.1, respectively. Land 2019, 8, x FOR PEER REVIEW 10 of 14 Figure Figure 7. 7. Ch Changes anges of of G GMSN MSN ((A A) ) and and SS SSAN AN ((B B) ) un under der d di iff er erent ent land land uses uses. . G GMSN MSN repr repres esent ent geom geometric etric m means eans of of LO LOC C and and avai available lable nu nutrients. trients. SS SSAN AN repr repres esent ent sum sum sc scor ores es of of LO LOC C and and avai available lable nu nutri trients ents. . The The bars barsr epr repr esent esent standar standard d err error ors. V s.alues Value without s without a common a comm letter on le within tter wit land hin use land treatments use treatm di er ents ed di accor ffered ding acto cord the ing to LSD the testLS (p D < tes 0.05). t (p NS < 0.= 05). not NS significant = not sign among ificant di am ong erent differen land uses. t land uses. 4. Discussion 4. Discussion The low content of soil organic carbon can limit microbial biomass and activity, nutrient cycling, The low content of soil organic carbon can limit microbial biomass and activity, nutrient cycling, soil structure formation, etc., and therefore indirectly limit plant growth [22]. Increasing the content of soil structure formation, etc., and therefore indirectly limit plant growth [22]. Increasing the content soil organic carbon and soil available nutrients is the common approach to improve soil productivity of soil organic carbon and soil available nutrients is the common approach to improve soil and agricultural sustainability. Land use changes could significantly alter the inputs and outputs of productivity and agricultural sustainability. Land use changes could significantly alter the inputs and soil organic matter, thus resulting in the variations in the content and circulation of soil labile carbon outputs of soil organic matter, thus resulting in the variations in the content and circulation of soil and soil nutrients [32,33]. The present study showed that conversion of cropland to revegetation labile carbon and soil nutrients [32,33]. The present study showed that conversion of cropland to land increased the LOC content in the 0–50 cm depth (Figure 4A). Moreover, the increase of LOC revegetation land increased the LOC content in the 0–50 cm depth (Figure 4A). Moreover, the increase content mainly occurred in the surface soil. Compared with the corn treatment, the LOC contents of LOC content mainly occurred in the surface soil. Compared with the corn treatment, the LOC under revegetation land were 33%, 33%, and 20% higher in the 0 to 10, 10 to 20, and 20 to 30 cm contents under revegetation land were 33%, 33%, and 20% higher in the 0 to 10, 10 to 20, and 20 to 30 depths, respectively. However, there were no significant di erences for LOC contents between corn cm depths, respectively. However, there were no significant differences for LOC contents between and revegetation treatments in the 30 to 40 and 40 to 50 cm depths. The higher LOC contents under corn and revegetation treatments in the 30 to 40 and 40 to 50 cm depths. The higher LOC contents revegetation land in surface soil (0–30 cm) were probably associated with the accumulation of above- under revegetation land in surface soil (0–30 cm) were probably associated with the accumulation of above- and below-ground biomass incorporated into the surface soils [34,35]. In addition, revegetation on the cropland could reduce the loss of LOC in fine soil fractions caused by rain and wind erosion, thus increasing the LOC content [22,33]. Soil texture can affect the soil aggregation processes and, therefore, influences the soil capacity to sequester organic carbon [36]. Tian et al. [37] in the alpine grassland on the Tibetan Plateau reported that soil organic carbon and total nitrogen Land 2020, 9, 10 11 of 14 and below-ground biomass incorporated into the surface soils [34,35]. In addition, revegetation on the cropland could reduce the loss of LOC in fine soil fractions caused by rain and wind erosion, thus increasing the LOC content [22,33]. Soil texture can a ect the soil aggregation processes and, therefore, influences the soil capacity to sequester organic carbon [36]. Tian et al. [37] in the alpine grassland on the Tibetan Plateau reported that soil organic carbon and total nitrogen stocks positively correlated with clay content and silt content, while they negatively related to sand content. Land use changes can indirectly a ect soil texture through the redistribution of soil by erosional processes or tillage. Revegetation on the cropland in this study could reduce the soil erosion by increasing the vegetation cover and decreasing soil disturbance, thus indirectly a ecting the content of LOC and soil available nutrients. Compared with corn treatment, revegetation did not increase the AN contents in the study area, and the corn treatment had the highest AN content in the 0 to 50 cm depth (Figure 4B). This might be due to the fertilization management in corn treatment, which applied approximately 74 kg N ha every year. Another reason for the higher AN contents under corn treatment could partially result from the short term revegetation under the revegetation land, which had limited e ects on the accumulation of AN and other soil nutrients. Besides, no significant di erences among the forage and grasslands also suggested negligible e ects of short term revegetation on the AN contents in the study area. The higher AN contents under the corn treatment were similar to the results by Zhang et al. [38] in Guizhou, China, who also reported that the AN content under fertilized and plowed cropland was higher than that under grassland and forestland. Soil AP and AK contents were not significantly di erent under most land use treatments (Figure 5), indicating that the short term land use treatments did not change the AP and AK contents in northeastern China. Similar to the changes in AN content, the negligible e ects of short term revegetation on soil AP and AK may be the primary reason for the narrow changes in AP and AK content under the five land use treatments. These results were in agreement with the findings of Zhao et al. [39] in another region of Songnen plain, who also found that the changes in AP and AK content under cropland and grassland were very limited. The SR of soil parameters was used as an indicator of the dynamics soil quality, and it could detect the management induced changes in the soil profiles of agricultural systems [15]. The increase in SR values of LOC and soil nutrient indicated the improvement of soil quality due to the accumulation of LOC and soil nutrients in the surface soil [30,40]. Land use treatments had no significant e ects on the SR values of LOC, AN, AP, and AK contents at depths of 0–10/10–20 cm and 0–10/20–30 cm except the SR of AP at the depth of 0–10/20–30 cm, suggesting that short term revegetation had limited e ects on the soil available nutrients in northeastern China. Studies in Columbia and Georgia showed that the SR values of SOC and total nitrogen were >2 under no tillage management, indicting an improvement of soil quality [29]. Peregrina et al. [41], Corral-Fernandez et al. [42], Francaviglia et al. [40], and Deng et al. [15] confirmed this finding, arguing that a high SR value (usually >2) indicated a better soil quality and contribution to agriculture sustainability. Our results showed that the SR values of LOC and soil available nutrients at the depth of 0–10/10–20 cm were mostly <2, and the SR values at the depth of 0–10/20–30 cm were mostly >2, indicating that soils under the same land use treatments had di erent soil quality. Similarly, the study by Deng et al. [15] also found that the SR values at the depth of 0–20/20–40 cm were generally higher than those at the depth of 0–5/10–20 cm found by Wang et al. [43] in the same region of the Loess Plateau. The SR values of LOC and soil nutrients in di erent soil depths in response to land use treatment were not consistent, suggesting that standard SR values of soil properties are needed in future studies to make the comparisons of soil quality under di erent management practices and di erent regions easier. Therefore, the SR values at the depth of 0–10/10–20 cm may be well suitable as a standard for evaluating significant changes in surface soils induced by management practices. In this study, three soil available nutrients including AN, AP, and AK contents and LOC were evaluated, but similar trends were not found (Figures 4 and 5). In fact, it is dicult to draw meaningful conclusions about soil quality changes when univariate indicators are used to analyze datasets involving Land 2020, 9, 10 12 of 14 many soil properties and reveal the changes within the soil environment [44]. The two indices of GMSN and SSAN were able to overcome the above weaknesses, and they were used as useful indictors of soil quality in other studies [22,26,31]. However, the results in this study showed no significant di erences of GMSN and SSAN among the five land use treatments at each soil depth except SSAN under the MLG + M treatment in the 0 to 10 cm depth (Figure 7), indicating that short term conversions of cropland to revegetation land had limited demonstrable influences on the soil available nutrients and LOC in the salt a ected region of Songnen plain. The inconclusive results suggested that a long term study is needed to examine the responses of the LOC and soil available nutrients to long term revegetation in northeastern China. 5. Conclusions The present results showed that revegetation on the cropland enhanced the LOC contents and decreased the AN contents in the 0–50 cm depth compared with the Corn treatment, and the changes in AP and AK contents were very limited after the land use conversions. The SR values in di erent soil depths in response to land uses were not consistent, suggesting that standard SR values of soil properties are needed in the future studies and that the SR values at the depth of 0–10:10–20 may be suitable as the standard considering the notable changes in surface soils induced by management practices. However, more studies are needed to examine if the SR value at the 0–10:10–20 cm is suitable in other managements or regions. The values of SR, GMSA, and SSAN were not a ected by the land use changes, indicating short term revegetation on the cropland had limited influences on the changes in soil nutrients and LOC in northeastern China. Compared with AG treatment, values of GMSA and SSAN were slightly lower than other land use treatments. These results were mainly due to the very short term (five years) revegetation because revegetation may need more time to be incorporated. Therefore, more studies are needed to assess the long term (more than 10 years) e ects of revegetation on soil properties in the Songnen grassland in the future. Although changes in soil available nutrients were given in this study, variations in soil microbial populations, which are more sensitive to changes in land uses than soil nutrients, were not mentioned. The influences of short term revegetation on soil quality need to be comprehensively assessed. In addition, we recommend that farmers in Northeast China should use revegetation to rehab grassland in areas with poor quality soils in the long run. Author Contributions: Conceptualization, P.Y., S.L., X.T., and W.L.; methodology, P.Y., X.T., and A.Z.; investigation, P.Y. and X.T.; data curation, P.Y. and A.Z.; writing, original draft preparation, P.Y.; writing, review and editing, S.L., W.L., X.T., and A.Z.; project administration, P.Y., S.L. and W.L.; funding acquisition, P.Y. and S.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Fundamental Research Funds for the Central Universities in China, Grant Numbers SWU019024 and SWU019023; the National Natural Science Foundation of China, Grant Number 41601124 and 31500446; the Excellent Young Foundation of Jilin Province, Grant Number 20190103141JH; and the University Innovation Research Group of Chongqing (Remote sensing of fragile ecological environment in southwest China). Conflicts of Interest: The authors declare no conflict of interest. References 1. Sardans, J.; Bartrons, M.; Margalef, O.; Gargallo-Garriga, A.; Janssens, I.A.; Ciais, P.; Obersteiner, M.; Sigurdsson, B.D.; Chen, H.Y.H.; Penuelas, J. Plant invasion is associated with higher plant-soil nutrient concentrations in nutrient-poor environments. Glob. Chang. Biol. 2017, 23, 1282–1291. [CrossRef] [PubMed] 2. Yu, P.J.; Liu, S.W.; Xu, Q.; Fan, G.H.; Huang, Y.X.; Zhou, D.W. 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Short Term Effects of Revegetation on Labile Carbon and Available Nutrients of Sodic Soils in Northeast China

Yu, Pujia; Tang, Xuguang; Liu, Shiwei; Liu, Wenxin; Zhang, Aichun
Land , Volume 9 (1) – Jan 2, 2020
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land Article Short Term E ects of Revegetation on Labile Carbon and Available Nutrients of Sodic Soils in Northeast China 1 , 2 , 3 1 , 3 1 , 2 , 3 2 , 4 Pujia Yu , Xuguang Tang , Shiwei Liu , Wenxin Liu * and Aichun Zhang Chongqing Jinfo Mountain Field Scientific Observation and Research Station for Karst Ecosystem, School of Geographical Sciences, Southwest University, Chongqing 400715, China; yupujia@swu.edu.cn (P.Y.); xgtang@swu.edu.cn (X.T.); liushiwei@swu.edu.cn (S.L.) Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, China State Cultivation Base of Eco-agriculture for Southwest Mountainous Land, Southwest University, Chongqing 400715, China College of Mobile Telecommunications, Chongqing University of Posts and Telecom, Chongqing 401520, China; zac12357@163.com * Correspondence: liuwx@iga.ac.cn; Tel.: +86-1874-301-9395 Received: 26 November 2019; Accepted: 30 December 2019; Published: 2 January 2020 Abstract: In response to land degradation and the decline of farmers’ income, some low quality croplands were converted to forage or grassland in Northeast China. However, it is unclear how such land use conversions influence soil nutrients. The primary objective of this study was to investigate the influences of short term conversion of cropland to alfalfa forage, monoculture Leymus chinensis grassland, monoculture Leymus chinensis grassland for hay, and successional regrowth grassland on the labile carbon and available nutrients of saline sodic soils in northeastern China. Soil labile oxidizable carbon and three soil available nutrients (available nitrogen, available phosphorus, and available potassium) were determined at the 0–50 cm depth in the five land uses. Results showed that the treatments of alfalfa forage, monoculture grassland, monoculture grassland for hay, and successional regrowth grassland increased the soil labile oxidizable carbon contents (by 32%, 28%, 15%, and 32%, respectively) and decreased the available nitrogen contents (by 15%, 19%, 34%, and 27%, respectively) in the 0–50 cm depth compared with cropland, while the di erences in the contents of available phosphorus and available potassium were less pronounced. No significant di erences in stratification ratios of soil labile carbon and available nutrients, the geometric means of soil labile carbon and available nutrients, and the sum scores of soil labile carbon and available nutrients were observed among the five land use treatments except the stratification ratio of 0–10/20–30 cm for available phosphorus and the values of the sum scores of soil labile carbon and available nutrients in the 0–10 cm depth. These findings suggest that short term conversions of cropland to revegetation have limited influences on the soil labile carbon and available nutrients of sodic soils in northeastern China. Keywords: revegetation; Solonetz; stratification ratio; geometric mean; Songnen plain 1. Introduction The structure, diversity, and production capacity of terrestrial ecosystems are strongly linked to the availability of soil nutrients, such as nitrogen, phosphorus, potassium, and soil organic carbon [1,2]. However, the soil labile carbon and soil nutrients’ availability in terrestrial ecosystems are usually influenced by various direct and indirect soil disturbances [3,4]. Land use conversions are major drivers of changes in soil labile carbon and soil nutrient availability, resulting in the degradation of soil ecosystem services (nutrient cycle, water conservation, pollution purification, etc.) and global Land 2020, 9, 10; doi:10.3390/land9010010 www.mdpi.com/journal/land Land 2020, 9, 10 2 of 14 environmental problems (soil degradation, climate change, water, soil erosion, etc.) [5,6]. One main mechanism by which they do this is by changing the quantity and quality of plant biomass supplied to the soils, a ecting the rate of organic matter decomposition and the activity of soil microorganism and redistributing soil carbon and nutrients within soil profiles [7–9]. Another main mechanism is the impact of soil erosion, which preferentially removes the surficial and most carbon and nutrient rich material, thus accelerating the decline of the soil organic carbon and soil nutrient pool [10,11]. Characterizing the spatial and temporal variability of soil carbon and nutrients in relation to land use types is critical for predicting the influences of future land use conversion on soil quality changes and understanding how ecosystems’ function [12]. Changes of soil organic carbon (soil microbial biomass carbon, particulate organic carbon, water extractable organic carbon, etc.) and total nutrients (total nitrogen, total phosphorus, total potassium, etc.) under di erent land uses in di erent spatial and temporal scales induced by long term land use conversions have been well addressed [13–17]. However, the evaluation of the short term e ects of land uses on soil labile carbon and soil available nutrients are rare due to the high spatial variability and the errors in the measurement methods of these soil properties [18,19]. Moreover, due to regional di erences in environment conditions, initial soil properties, and management years and intensity, inconsistent and contradictory responses of soil labile carbon and soil available nutrients to short term land use conversions were observed, which failed to demonstrate a clear relationship between short term land uses and changes in soil labile carbon and available nutrients [14,20]. For instance, Madejon et al. [14] in the southwest of Spain found that after three year plantation of fast growing trees decreased the contents of available nitrogen (AN), available phosphorus (AP), and available potassium (AK). Lu et al. [21] in Tibet, China, reported that short term (nine years) grazing exclusion had no impact on soil AN, AP, microbial biomass carbon, and other soil properties. However, the results of Wang et al. [20] in Shanxi Province, China, showed that three years of plantation of grass and alfalfa significantly increased the contents of soil organic carbon, AN, AP, and AK. As one of the largest salt a ected soil regions in China, Songnen plain has su ered from substantial land salinization and alkalization because of the influences of human activity in recent decades [22]. The increases in soil alkalinity and sodicity have adversely influenced the soil properties by promoting crusting and low permeability and infiltration rates [23], thus leading to the reduction of grain production. Furthermore, with an increase in corn production in China from 1.06 10 tons in 2003 to 2.25  10 tons in 2016 due to the increase of yield per unit and the planting area, oversupply resulted, causing a notable reduction in corn price and planting benefit to farmers [22]. Therefore, the croplands with poor quality soils were abandoned in the Songnen plain. To address these problems in the Songnen plain, the Chinese government implemented a range of policies and subsidies to guide farmers to improve the eciency and sustainable development of agriculture through revegetation in the areas where the soils were not suitable for growing crops. Revegetation, the conversion of cropland to a vegetation covered land, has become a well explored approach to rehabilitate degraded soil ecosystems [15,24]. In addition to combatting erosion and protect soils, revegetation has substantial e ects on the accumulation of soil organic carbon and nutrients and the improvement of soil microbial biomass and activity [13,15]. Deng et al. [15] in the Loess Plateau, China, found that the contents of soil organic carbon, total N, and total P at a depth of 0–20 cm increased by more than 13%, 10% and 11%, respectively, after 30 years of grassland restoration. Our previous study in the same site showed that soil microbial biomass carbon and enzyme activity at a depth of 0–20 cm increased by 36% and 56%, respectively, after five years of conversion of cropland to grassland [22]. However, the short term influences of revegetation on soil oxidizable carbon and available nutrients have yet to be quantified in northeastern China. In this study, we hypothesized that five years of revegetation from cropland could increase the contents of soil available nutrients and soil labile carbon and therefore be beneficial to the sustainable use of saline sodic soils in northeastern China. To address this hypothesis, the objective of this research was to investigate the changes in the contents of labile oxidizable carbon (LOC), AN, AP, and AK Land 2019, 8, x FOR PEER REVIEW 3 of 14 research was to investigate the changes in the contents of labile oxidizable carbon (LOC), AN, AP, Land 2020, 9, 10 3 of 14 and AK after conversion from cropland to alfalfa forage, monoculture Leymus chinensis grassland, monoculture Leymus chinensis grassland for hay, and successional regrowth grassland and examine after conversion from cropland to alfalfa forage, monoculture Leymus chinensis grassland, monoculture whether short term revegetation could improve the LOC and soil available nutrients in the regions Leymus chinensis grassland for hay, and successional regrowth grassland and examine whether short where the soils were not suitable for planting crops in northeastern China. term revegetation could improve the LOC and soil available nutrients in the regions where the soils were not suitable for planting crops in northeastern China. 2. Materials and Methods 2. Materials and Methods 2.1. Study Area 2.1. Study Area The research was conducted in the Songnen plain located at the Grassland Farming and Ecological Research Station (123°31′ E, 44°33′ N) (Figure 1). The terrain surrounding the study area is The research was conducted in the Songnen plain located at the Grassland Farming and Ecological relatively flat, and the altitude is approximately 145 m above sea level. The study area has a 0  0 Research Station (123 31 E, 44 33 N) (Figure 1). The terrain surrounding the study area is relatively temperate, semiarid continental climate. The average annual temperature is 5.9 °C, and the mean flat, and the altitude is approximately 145 m above sea level. The study area has a temperate, semiarid annual precipitation is 427 mm (1980–2013). The soil is classified as Solonetz in the World Reference continental climate. The average annual temperature is 5.9 C, and the mean annual precipitation is Base for Soil Resources with a soil texture of 22% sand, 33% silt, and 45% clay [25]. The main 427 mm (1980–2013). The soil is classified as Solonetz in the World Reference Base for Soil Resources vegetation consists of perennial herbs such as Leymus chinensis and Puccinellia tenuiflora. Besides, some with a soil texture of 22% sand, 33% silt, and 45% clay [25]. The main vegetation consists of perennial therophytes such as Chloris virgata and Suaeda heteroptera grow in the areas with higher soil pH and herbs such as Leymus chinensis and Puccinellia tenuiflora. Besides, some therophytes such as Chloris virgata poor soil quality [26]. and Suaeda heteroptera grow in the areas with higher soil pH and poor soil quality [26]. Figure 1. The location map of the study area. Figure 1. The location map of the study area. 2.2. Experimental Design 2.2. Experimental Design This experiment was organized as a completed block design with five land use treatments. In early This experiment was organized as a completed block design with five land use treatments. In May 2011, four adjacent blocks (each 60 50 m, 2 m bu er between the blocks) in the study area based early May 2011, four adjacent blocks (each 60 × 50 m, 2 m buffer between the blocks) in the study area on similar land use history were identified. Before this experiment (2004–2010), farmers grew rain fed based on similar land use history were identified. Before this experiment (2004–2010), farmers grew maize (Zea mays L.) and sunflower (Helianthus annuus) in these blocks, following the traditional planting rain fed maize (Zea mays L.) and sunflower (Helianthus annuus) in these blocks, following the practices in Northeast China, which consists of plowing the soil down to 20 cm depth and applying traditional planting pr 1actices in Northe 1ast China, which con1sists of plowing the soil down to 20 cm 50–96 kg N ha , 20–45 kg P ha , and 15–45 kg K ha fertilizers into the soils. The soil properties in −1 −1 −1 depth and applying 50–96 kg N ha , 20–45 kg P ha , and 15–45 kg K ha fertilizers into the soils. The these four blocks were homogeneous due to the continuous plowing. The five land use treatments soil properties in these four blocks were homogeneous due to the continuous plowing. The five land consisted of corn cropland (corn, used as an indication of how the revegetation influences the soils in use treatments consisted of corn cropland (corn, used as an indication of how the revegetation this study), alfalfa perennial forage land (alfalfa), monoculture grassland of Leymus chinensis (MLG), influences the soils in this study), alfalfa perennial forage land (alfalfa), monoculture grassland of monoculture grassland of Leymus chinensis for hay (Mowing) once a year (MLG + M), and successional Leymus chinensis (MLG), monoculture grassland of Leymus chinensis for hay (Mowing) once a year regrowth grassland (SRG) (Figure 2). Leymus chinensis is the native vegetation and is usually used as (MLG + M), and successional regrowth grassland (SRG) (Figure 2). Leymus chinensis is the native forage grass for grazing animals in the Songnen grassland. In the mowing grassland, Leymus chinensis vegetation and is usually used as forage grass for grazing animals in the Songnen grassland. In the is harvested as hay, and farmers sell the hay to livestock farms. Alfalfa has high saline and alkaline mowing grassland, Leymus chinensis is harvested as hay, and farmers sell the hay to livestock farms. tolerance, and it has been introduced into the Songnen grassland as a high forage plant due to the Alfalfa has high saline and alkaline tolerance, and it has been introduced into the Songnen grassland high N and protein content [22]. The planting of forage grass used in this study could improve the as a high forage plant due to the high N and protein content [22]. The planting of forage grass used income of local farmers and the development of animal husbandry. In each block, two greater plots of in this study could improve the income of local farmers and the development of animal husbandry. 12 50 m were for corn and alfalfa treatments, while three plots of 6 50 m for land use treatments of MLG, MLG + M, and SRG. There was a 1 m bu er among the five plots. The was no irrigation under Land 2019, 8, x FOR PEER REVIEW 4 of 14 Land 2019, 8, x FOR PEER REVIEW 4 of 14 Land 2020, 9, 10 4 of 14 In each block, two greater plots of 12 × 50 m were for corn and alfalfa treatments, while three plots of In each block, two greater plots of 12 × 50 m were for corn and alfalfa treatments, while three plots of 6 × 50 m for land use treatments of MLG, MLG + M, and SRG. There was a 1 m buffer among the five 6 × 50 m for land use treatments of MLG, MLG + M, and SRG. There was a 1 m buffer among the five the five land uses in this study. More information about the treatments of land uses is presented in plots. The was no irrigation under the five land uses in this study. More information about the plots. The was no irrigation under the five land uses in this study. More information about the Figure 3 [22,26]. treatments of land uses is presented in Figure 3 [22,26]. treatments of land uses is presented in Figure 3 [22,26]. Figure 2. Images of the land use treatments in this study. Corn, corn cropland; Alfalfa, alfalfa forage Figure 2. Images of the land use treatments in this study. Corn, corn cropland; Alfalfa, alfalfa forage Figure 2. Images of the land use treatments in this study. Corn, corn cropland; Alfalfa, alfalfa land; MLG, monoculture grassland; MLG + M, monoculture grassland for hay (Mowing); SRG, land; MLG, monoculture grassland; MLG + M, monoculture grassland for hay (Mowing); SRG, forage land; MLG, monoculture grassland; MLG + M, monoculture grassland for hay (Mowing); SRG, successional regrowth grassland. successional successionalr re egr gr owth owth grassland. grassland. Successional Monoculture Monoculture Alfalfa perennial forage Corn cropland (Corn) Successional Monoculture Monoculture Alfalfa perennial forage Corn cropland (Corn) regrowth grassland grassland of Leymus grassland of Leymus (Alfalfa) regrowth grassland grassland of Leymus grassland of Leymus (Alfalfa) (SRG) chinensis for hay chinensis (MLG) Since 2011, the cropland (SRG) chinensis for hay chinensis (MLG) Since 2011, the cropland (MLG + M) Before 2014, the Alfalfa has been under continuous (MLG + M) Before 2014, the Alfalfa has been under continuous Seeds of Leymus plots were no tillage corn monoculture. The The cropland was Seeds of Leymus plots were no tillage corn monoculture. The The cropland was chinensis (Trin.) cropland, and other Corn plots followed the abandoned in 2011 in Seeds of Leymus chinensis (Trin.) cropland, and other Corn plots followed the abandoned in 2011 in Seeds of Leymus Tzvelev were sowed practices were the same as traditional cropland the SRG plots to chinensis (Trin.) Tzvelev were sowed practices were the same as traditional cropland the SRG plots to chinensis (Trin.) in May 2011 with a those of the previously practice in the Songnen restore grassland Tzvelev were sowed in May 2011 with a those of the previously practice in the Songnen restore grassland Tzvelev were sowed density of described corn cropland. grassland; this tradition without any in May 2011 with a density of described corn cropland. grassland; this tradition without any in May 2011 with a approximately 2000 However, the growth of consists of plowing the disturbance. The density of approximately 2000 However, the growth of consists of plowing the disturbance. The density of −2 seeds m . Reseeding corn was very poor in 2011 soil at least twice before dominant species in approximately 2000 −2 seeds m . Reseeding corn was very poor in 2011 soil at least twice before dominant species in approxim 2 ately 2000 had a positive effect to 2013 due to the poor soil the crop growing season the SRG plots include seeds/m . Reseeding had a positive effect to 2013 due to the poor soil the crop growing season the SRG plots include seeds/m . Reseeding on the recovery of conditions and short term down to 20 cm and Chloris virgate, had a positive effect in −1 on the recovery of conditions and short term down to 20 cm and Chloris virgate, had a positive effect in vegetation, and the land use, and the no tillage fertilization (74 kg N ha , Sonchus brachyotus, recovering the −1 −1 vegetation, and the land use, and the no tillage fertilization (74 kg N ha , Sonchus brachyotus, recovering the aboveground biomass cropland was changed to 22 kg P ha , and 41 kg K Chenopodium vegetation. The −1 −1 aboveground biomass cropland was changed to 22 kg P ha , and 41 kg K Chenopodium vegetation. The reached alfalfa forage land in May ha ) twice per year at glaucum, etc. The aboveground biomass −1 reached alfalfa forage land in May ha ) twice per year at glaucum, etc. The aboveground biomass approximately 100– 2014 with a sowing density sowing and in mid-July. aboveground (348 g was mowed for hay −2 −2 approximately 100– 2014 with a sowing density sowing and in mid-July. aboveground (348 g was mowed for hay 120 g m in early of approximately 1200 The corn straw was m ) and belowground once a year at the peak −2 −2 −2 −2 120 g m in early of approximately 1200 The corn straw was m ) and belowground once a year at the peak September 2011. The seeds m . The removed from the plots (397 g m , 0–20 cm biomass. The −2 −2 −2 September 2011. The seeds m . The removed from the plots (397 g m , 0–20 cm biomass. The aboveground (381 g aboveground (307 g m ) after harvest, while the depth) biomass was belowground (638 g −2 −2 −2 aboveground (381 g aboveground (307 g m ) after harvest, while the depth) biomass was belowground (638 g m ) and belowground and belowground (321 g corn root, stem base, and kept as litter to return m , 0–20 cm depth) −2 −2 −2 −2 m ) and belowground and belowground (321 g corn root, stem base, and (456 g m , 0–20 cm m , 0–20 cm depth) aerial root (which is 137 g kto ep the so t as litte il. r to return m biom , ass 0–2 0 was cmk ep detp th to) −2 −2 −2 d (45 epth 6 ) g b m iom , ass 0– 20 was cm bio mm, ass we 0–2re 0 kep cm t in th dese epth ) m aeria ) we l ro re ot (which incorpo irated s 137 g to the soil. b retu iom rn ass to was the so kep ils t in to −2 kept as litter to return plots to increase soil into soil during plowing. depth) biomass was biomass were kept in these m ) were incorporated retu these p rn lo tots. th e soils in to the soil. fertility in 2014 and 2015. kept as litter to return plots to increase soil into soil during plowing. these plots. to the soil. fertility in 2014 and 2015. Figure 3. Detailed information and history of land use treatments in this study. Figure 3. Detailed information and history of land use treatments in this study. Figure 3. Detailed information and history of land use treatments in this study. 2.3. Soil Sampling and Analysis 2.3. Soil Sampling and Analysis 2.3. Soil Sampling and Analysis Soil sampling was performed using an auger (4 cm in diameter) in early September 2015. Soil sampling was performed using an auger (4 cm in diameter) in early September 2015. The The sampling depth was 0–50 cm with an interval of 10 cm increments. Five randomly distributed Soil sampling was performed using an auger (4 cm in diameter) in early September 2015. The sampling depth was 0–50 cm with an interval of 10 cm increments. Five randomly distributed sub- sub-samples from each plot were combined into a composite sample at each soil depth. After removing sa sa mp mp ling les fr dep omth ea was ch plot 0–50 we cre m com with bined an interv into al a com of 10 poc sm ite incremen sample at tsea . F ch ive soril ando depmly th. After distrrem ibuted oving sub- the visible vegetation materials and debris, soil samples were sieved through a 2 mm sieve, and then sa th mp e visi les bfr le om vegetation each plot m ate werials re com and bined debris int , soi o a l com samp po les site were samp sieve le d at th ea rough ch soil a dep 2 mm th . siAfter eve, an rem d th ov en ing ground to pass through a 0.25 mm sieve for analyses. ground to pass through a 0.25 mm sieve for analyses. the visible vegetation materials and debris, soil samples were sieved through a 2 mm sieve, and then Soil labile oxidizable carbon (LOC) was measured using the revised method defined by ground Soil to pa labile ss oxi throu dizabl gh a 0.25 e carbon mm (LOC) sieve f wor as a meas nalyse ured s. using the revised method defined by Chan et Chan et al. [27]. Available nitrogen (AN) was measured by the alkaline hydrolysis di usion method [28]. al. [27]. Available nitrogen (AN) was measured by the alkaline hydrolysis diffusion method [28]. The Soil labile oxidizable carbon (LOC) was measured using the revised method defined by Chan et The AN AN forms forms were wer prim e primarily arily mixtmixtur ures of es am of mo ammonium nium nitrogen nitrogen (NH3-(NH N), nitr -N), ate nitrate nitrogen nitr (NO ogen 3-N) (NO , and -N), a al. [27]. Available nitrogen (AN) was measured by the alkaline hydrolysis 3 diffusion method [28] 3. The and small a small amoun amount t of water of water soluble soluble organic organic nitrogen nitr(e ogen .g., amino (e.g., amino acids and acids am and moniu ammonium m acyl, etc acyl, .). AN forms were primarily mixtures of ammonium nitrogen (NH3-N), nitrate nitrogen (NO3-N), and a Available phosphorus (AP) was extracted with NaHCO3 at pH 8.5 and measured using UV etc.). Available phosphorus (AP) was extracted with NaHCO at pH 8.5 and measured using UV small amount of water soluble organic nitrogen (e.g., amino 3 acids and ammonium acyl, etc.). spectrophotometer [28]. The AP forms were primarily the calcium phosphates due to the higher soil spectrophotometer [28]. The AP forms were primarily the calcium phosphates due to the higher soil Available phosphorus (AP) was extracted with NaHCO3 at pH 8.5 and measured using UV pH (Table 1) in the study area. Available potassium (AK) was measured based on the ammonium pH (Table 1) in the study area. Available potassium (AK) was measured based on the ammonium spectrophotometer [28]. The AP forms were primarily the calcium phosphates due to the higher soil acetate extracted and emission flame spectrophotometer method [28]. The AK forms were primarily pH (Table 1) in the study area. Available potassium (AK) was measured based on the ammonium Land 2020, 9, 10 5 of 14 mixtures of exchangeable potassium and water soluble potassium. Furthermore, for the purpose of clearly understanding the nature of soils in the study area, a 1:5 soil:water solution was used to measure the soil pH and electrical conductivity (EC) using the PHS-3C instrument and the DDS-307 instrument, respectively. Table 1. Mean values (SE) of soil pH and electrical conductivity (EC) under di erent land uses. Soil Corn Alfalfa MLG + M MLG SRG ANOVA Depth F P pH 0–10 9.36 (0.14) 9.14 (0.04) 9.16 (0.10) 9.00 (0.10) 9.05 (0.26) 0.88 0.50 10–20 9.83 (0.17) 9.76 (0.09) 9.93 (0.09) 9.87 (0.06) 9.39 (0.43) 1.01 0.43 20–30 10.06 (0.08) 10.04 (0.09) 10.03 (0.04) 10.00 (0.06) 9.96 (0.08) 0.29 0.88 30–40 10.11 (0.04) 10.11 (0.08) 10.03 (0.03) 9.99 (0.04) 10.02 (0.03) 1.11 0.39 40–50 10.06 (0.06) 10.04 (0.09) 9.99 (0.04) 9.92 (0.04) 9.98 (0.05) 0.78 0.56 EC 0–10 204 (16) 155 (11) 162 (5) 157 (21) 164 (24) 1.43 0.27 10–20 330 (56) 306 (43) 398 (53) 376 (32) 335 (101) 0.36 0.84 20–30 391 (49) 385 (65) 531 (55) 491 (39) 435 (95) 1.00 0.44 30–40 389 (31) 391 (58) 576 (46) 501 (55) 492 (75) 2.12 0.13 40–50 350 (17) 349 (59) 481 (56) 468 (64) 452 (45) 1.63 0.22 Abbreviations: Corn, corn cropland; Alfalfa, alfalfa forage land; MLG, monoculture grassland; MLG + M, monoculture grassland for hay; SGR, successional growth grassland. 2.4. Statistical Analysis The soil stratification by certain properties (e.g., SOC, total N, total P, etc.) is very common, and the stratification ratio (SR) is widely used as a crucial indicator of soil condition [15]. A higher SR of soil properties indicates better soil conditions, because SR of degraded soils is usually less than 2 regardless of climatic or soil conditions [15]. The improvement of soil quality under specific land use is conducive to plant growth and agricultural sustainability [29,30]. Revegetation on the cropland will increase the input of organic matter and thus alter the SR of soil properties, which will provide an indication of soil responses to specific plant cover. The SR were calculated for each land use as follows: SR = ANt/ANs (1) where AN is the content of LOC, AN, AP, and AK in the 0–10 cm depth; AN is the corresponding t s content of LOC, AN, AP, and AK in the 10–20 and 20–30 cm depth. A unitary soil available nutrient is not complete to reveal the changes within the soil environment because soil available nutrients do not always respond similarly to di erent management practices [22]. Therefore, the comprehensive assessment of the responses of a series of soil available nutrients and LOC to factors of change is required. However, the various responses of LOC and soil available nutrients to land use change might result in inaccurate conclusions on soil quality and thus limit the suitability of LOC and soil available nutrients as soil quality indicators. The geometric mean and sum scores are two general indices to combine the variables with diverse units and ranges into one variable, which could clearly indicate the actual influences of environmental factor changes on these variables [31]. Here, the geometric means of LOC and available nutrients (GMSN) under di erent land uses and soil depths are calculated as follows: 1/4 GMSN = (LOC AN AP AK) (2) where LOC, AN, AP, and AK are oxidizable labile C, available nitrogen, available phosphorus, and available potassium, respectively. Land 2020, 9, 10 6 of 14 The simple sum of the series of soil available nutrients and LOC with di erent units and ranges of variation may cover up the changes in some soil nutrients. Therefore, data normalization is needed for all the measured soil available nutrients and LOC before the sum scores of LOC and available nutrients (SSAN) are calculated. The min-normalization is a well explored approach to convert the data with di erent units or variation ranges into a dimensionless pure value, so that the data can remove the unit limit and can be easily compared and weighted [26]. The SSAN under di erent land uses and soil depths is as follows: S = X/X (3) i min SSAN = S (4) i=1 where S is the score of LOC, AN, AP, and AK after data normalization; X is the measured value, and X is the minimum value of each soil nutrient observed in this study; n is the number of soil nutrients. min We used one way ANOVA to analyze the influences of land use types on the LOC and soil available nutrients, soil pH, EC, SR, GMSN, and SSAN. Mean di erences of soil available nutrient contents, LOC, SR, GMSN, and SSAN among land use treatments were examined using the least significant di erence test (LSD). All comparisons were considered significant if p < 0.05. The mean and standard error of each soil property measured were provided at each soil depth under a given land use treatment. All data analyses were performed with SPSS 16.0 for Windows (SPSS, Inc., Chicago, USA). 3. Results 3.1. Changes in Soil pH and EC Soil pH in the study area was notably high (Table 1). The values of soil pH were all more than 9.00 at the 0–50 cm depth; especially at the 10–50 cm depths, the values were close to or more than 10.00. Soil pH was not a ected by the land use conversions. The average values of soil pH at the 0–50 cm depth were 9.88, 9.82, 9.83, 9.76, and 9.68 for corn, alfalfa, MLG, MLG + M, and SRG treatment, respectively. Similar to the soil pH, the EC values in the subsoil (10–50 cm) were higher than that at the surface soil (0–10 cm). The average values of EC in the 0–50 cm depth were 333, 317, 430, 399, and 376 S -1 cm for corn, alfalfa, MLG, MLG + M, and SRG treatment, respectively (Table 1). There was no significant di erence of EC among the land use treatments because of the narrow values of EC in the same soil depth. 3.2. Changes in LOC, AN, AP, and AK Content The LOC content under the land use of SRG was remarkably higher than that under corn in the 0–10 cm depth, while it was significantly higher under alfalfa in the 10–20 cm depth than the corn and MLG + M treatment (Figure 4A). In the 20–50 cm depth, the highest LOC content was found under the MLG treatment. The average LOC contents in the 0–50 cm depth were 32% (0.56 g kg ), 28% 1 1 1 (0.49 g kg ), 15% (0.26 g kg ), and 32% (0.57 g kg ) higher under alfalfa, MLG, MLG + M, and SRG treatment, respectively, than that under corn treatment. Land use conversions significantly (F = 8.76, p = 0.001) changed the AN contents (Figure 4B). The highest AN content was found under corn treatment in the 0–50 cm depth. In addition, the AN contents under corn treatment in the 20–30 cm and 40–50 cm depth were significantly higher than those under all the revegetation land except the alfalfa treatment in the 20–30 cm depth. The average 1 1 1 AN contents in the 0–50 cm depth were 15% (6.3 mg kg ), 19% (8.0 mg kg ), 34% (14.9 mg kg ), and 27% (11.8 mg kg ) lower under alfalfa, MLG, MLG + M, and SRG treatment, respectively, than under corn treatment. Land 2020, 9, 10 7 of 14 Land 2019, 8, x FOR PEER REVIEW 7 of 14 -1 LOC (g kg ) 0 1 2 3 4 5 6 7 8 ab 10 ab ab 20 ab ab NS ab Corn 40 a ab Alfalfa MLG MLG + M ab SRG ab -1 AN (mg kg ) 0 20 40 60 80 100 ab ab ab ab 20 ab ab Corn 40 NS Alfalfa MLG MLG + M SRG 50 bc bc Figure 4. Mean values of labile oxidizable carbon (LOC) (A) and available nitrogen (AN) (B) under Figure 4. Mean values of labile oxidizable carbon (LOC) (A) and available nitrogen (AN) (B) under di erent land uses. The bars represent standard errors. Values without a common letter within land different land uses. The bars represent standard errors. Values without a common letter within land use treatments di ered according to the LSD test (p < 0.05). NS = not significant among di erent land use treatments differed according to the LSD test (p < 0.05). NS = not significant among different land uses. See Figure 2 for the abbreviations. uses. See Figure 2 for the abbreviations. The di erences of AP and AK contents among the soil depths in the 0–50 cm depth were very The differences of AP and AK contents among the soil depths in the 0–50 cm depth were very narrow except the 0–10 cm depth (Figure 5). Compared with the land uses of MLG, MLG + M, and SRG, narrow except the 0–10 cm depth (Figure 5). Compared with the land uses of MLG, MLG + M, and land use of corn had a higher AP content at the 0–10 cm depth. However, land use treatments did not SRG, land use of corn had a higher AP content at the 0–10 cm depth. However, land use treatments change the AP contents at the 10–50 cm depth (Figure 5A). The average AP contents in the 0–50 cm did not change the AP contents at the 10–50 cm depth (Figure 5A). The average AP contents in the 0– depth under the land uses of corn, alfalfa, MLG, MLG + M, and SRG were 4.1, 3.8, 3.4, 3.1, and 3.4 mg 50 cm depth under the land uses of corn, alfalfa, MLG, MLG + M, and SRG were 4.1, 3.8, 3.4, 3.1, and −1 3.4 mg kg , respectively. The highest AK contents were all found under the MLG treatment in the 0– Soil depth (cm) Soil depth (cm) Land 2020, 9, 10 8 of 14 Land 2019, 8, x FOR PEER REVIEW 8 of 14 kg , respectively. The highest AK contents were all found under the MLG treatment in the 0–50 cm 50 depth cm d (Figur epth e (Fi 5B). gure However 5B). Howev , significant er, sign di ifi cer ant ences differe wer nces e only wer found e onlbetween y found bet MLG ween andMLG corn tr an eatment d corn in the 20–40 cm depth and between MLG and alfalfa treatment in the 30–40 cm depth. The average AK treatment in the 20–40 cm depth and between MLG and alfalfa treatment in the 30–40 cm depth. The aver contents age AK in the cont 0–50 ents cm in th depth e 0–50 under cm dep corn, th under alfalfa, corn MLG, , alfalfa MLG , M +L M, G, and MLG SRG + Mtr , eatment and SRGwer treatmen e 102.9, t −1 112.2, 137.5, 108.7, and 125.8 mg kg , respectively. were 102.9, 112.2, 137.5, 108.7, and 125.8 mg kg , respectively. -1 AP (mg kg ) 0 2 4 6 8 10 12 Corn Alfalfa MLG MLG + M SRG 0-10 cm: MLG, MLG + M, SRG < Corn Other soil depths: NS -1 AK (mg kg ) 0 50 100 150 200 250 300 350 400 Corn Alfalfa MLG MLG + M SRG 20-30 cm: Corn < MLG 30-40 cm: Corn, Alfalfa < MLG Other soil depths: NS Figure 5. Mean values of AP (A) and AK (B) under di erent land uses. AP represents available Figure 5. Mean values of AP (A) and AK (B) under different land uses. AP represents available phosphorus. AK represents available potassium. The bars represent standard errors. NS = not phosphorus. AK represents available potassium. The bars represent standard errors. NS = not significant among di erent land uses. significant among different land uses. 3.3. Changes in SR, GMSN, and SSAN The SR of LOC, AN, and AK in the 0–10/10–20 cm and in the 0–10/20–30 cm (Figure 6) were not affected by the different land uses. Land uses of corn and alfalfa had remarkably higher SR values of Soil depth (cm) Soil depth (cm) Land 2020, 9, 10 9 of 14 3.3. Changes in SR, GMSN, and SSAN Land 2019, 8, x FOR PEER REVIEW 9 of 14 The SR of LOC, AN, and AK in the 0–10/10–20 cm and in the 0–10/20–30 cm (Figure 6) were not a ected by the di erent land uses. Land uses of corn and alfalfa had remarkably higher SR values of AP AP in the 0–10/20–30 cm depth than the land uses of MLG, MLG + M, and SRG. However, the in the 0–10/20–30 cm depth than the land uses of MLG, MLG + M, and SRG. However, the di erences differences of the SR value for AP in the 0–10/10–20 cm depth were not significant among the five of the SR value for AP in the 0–10/10–20 cm depth were not significant among the five treatments. treatments. All the SR values of LOC, AN, AP, and AK in the 0–10/10–20 cm depth were all <2 except All the SR values of LOC, AN, AP, and AK in the 0–10/10–20 cm depth were all <2 except the values the values of AN under SRG treatment, AP under corn and alfalfa treatment, and AK under MLG of AN under SRG treatment, AP under corn and alfalfa treatment, and AK under MLG treatment. treatment. However, the SR values of LOC, AN, AP, and AK in the 0–10/20–30 cm depth were all >2, However, the SR values of LOC, AN, AP, and AK in the 0–10/20–30 cm depth were all >2, except the except the values of AN under corn treatment and AP under MLG and MLG + M treatment. values of AN under corn treatment and AP under MLG and MLG + M treatment. Figure 6. Changes in the stratification ratio (SR) of 0–10/10–20 cm (A) and 0–10/20–30 cm (B) under Figure 6. Changes in the stratification ratio (SR) of 0–10/10–20 cm (A) and 0–10/20–30 cm (B) under di erent land uses. The bars represent standard errors. Values without a common letter within land use different land uses. The bars represent standard errors. Values without a common letter within land treatments di ered according to the LSD test (p < 0.05). NS = not significant among di erent land uses. use treatments differed according to the LSD test (p < 0.05). NS = not significant among different land uses. Values of GMSN in the 0–50 cm depth and SSAN in the 10–50 cm depth were not influenced by the changes of land use (Figure 7). The highest GMSN value was found under alfalfa in 0–20 cm depth, Values of GMSN in the 0–50 cm depth and SSAN in the 10–50 cm depth were not influenced by while it was highest under MLG treatment in the 20–50 cm depth (Figure 7A). The average GMSN the changes of land use (Figure 7). The highest GMSN value was found under alfalfa in 0–20 cm values in the 0–50 cm depth under corn, alfalfa, MLG, MLG + M, and SRG treatment were 13.1, 13.2, depth, while it was highest under MLG treatment in the 20–50 cm depth (Figure 7A). The average 13.3, 11.2, and 12.7, respectively. The SSAN values under corn and alfalfa treatment in the 0–10 cm GMSN values in the 0–50 cm depth under corn, alfalfa, MLG, MLG + M, and SRG treatment were 13.1, 13.2, 13.3, 11.2, and 12.7, respectively. The SSAN values under corn and alfalfa treatment in the 0–10 cm depth were markedly greater than the land use of MLG + M (Figure 7B). The average SSAN values in the 0–50 cm depth under corn, alfalfa, MLG, MLG + M, and SRG treatment were 9.5, 9.0, 9.7, 8.6, and 9.1, respectively. Land 2020, 9, 10 10 of 14 depth were markedly greater than the land use of MLG + M (Figure 7B). The average SSAN values in the 0–50 cm depth under corn, alfalfa, MLG, MLG + M, and SRG treatment were 9.5, 9.0, 9.7, 8.6, and 9.1, respectively. Land 2019, 8, x FOR PEER REVIEW 10 of 14 Figure Figure 7. 7. Ch Changes anges of of G GMSN MSN ((A A) ) and and SS SSAN AN ((B B) ) un under der d di iff er erent ent land land uses uses. . G GMSN MSN repr repres esent ent geom geometric etric m means eans of of LO LOC C and and avai available lable nu nutrients. trients. SS SSAN AN repr repres esent ent sum sum sc scor ores es of of LO LOC C and and avai available lable nu nutri trients ents. . The The bars barsr epr repr esent esent standar standard d err error ors. V s.alues Value without s without a common a comm letter on le within tter wit land hin use land treatments use treatm di er ents ed di accor ffered ding acto cord the ing to LSD the testLS (p D < tes 0.05). t (p NS < 0.= 05). not NS significant = not sign among ificant di am ong erent differen land uses. t land uses. 4. Discussion 4. Discussion The low content of soil organic carbon can limit microbial biomass and activity, nutrient cycling, The low content of soil organic carbon can limit microbial biomass and activity, nutrient cycling, soil structure formation, etc., and therefore indirectly limit plant growth [22]. Increasing the content of soil structure formation, etc., and therefore indirectly limit plant growth [22]. Increasing the content soil organic carbon and soil available nutrients is the common approach to improve soil productivity of soil organic carbon and soil available nutrients is the common approach to improve soil and agricultural sustainability. Land use changes could significantly alter the inputs and outputs of productivity and agricultural sustainability. Land use changes could significantly alter the inputs and soil organic matter, thus resulting in the variations in the content and circulation of soil labile carbon outputs of soil organic matter, thus resulting in the variations in the content and circulation of soil and soil nutrients [32,33]. The present study showed that conversion of cropland to revegetation labile carbon and soil nutrients [32,33]. The present study showed that conversion of cropland to land increased the LOC content in the 0–50 cm depth (Figure 4A). Moreover, the increase of LOC revegetation land increased the LOC content in the 0–50 cm depth (Figure 4A). Moreover, the increase content mainly occurred in the surface soil. Compared with the corn treatment, the LOC contents of LOC content mainly occurred in the surface soil. Compared with the corn treatment, the LOC under revegetation land were 33%, 33%, and 20% higher in the 0 to 10, 10 to 20, and 20 to 30 cm contents under revegetation land were 33%, 33%, and 20% higher in the 0 to 10, 10 to 20, and 20 to 30 depths, respectively. However, there were no significant di erences for LOC contents between corn cm depths, respectively. However, there were no significant differences for LOC contents between and revegetation treatments in the 30 to 40 and 40 to 50 cm depths. The higher LOC contents under corn and revegetation treatments in the 30 to 40 and 40 to 50 cm depths. The higher LOC contents revegetation land in surface soil (0–30 cm) were probably associated with the accumulation of above- under revegetation land in surface soil (0–30 cm) were probably associated with the accumulation of above- and below-ground biomass incorporated into the surface soils [34,35]. In addition, revegetation on the cropland could reduce the loss of LOC in fine soil fractions caused by rain and wind erosion, thus increasing the LOC content [22,33]. Soil texture can affect the soil aggregation processes and, therefore, influences the soil capacity to sequester organic carbon [36]. Tian et al. [37] in the alpine grassland on the Tibetan Plateau reported that soil organic carbon and total nitrogen Land 2020, 9, 10 11 of 14 and below-ground biomass incorporated into the surface soils [34,35]. In addition, revegetation on the cropland could reduce the loss of LOC in fine soil fractions caused by rain and wind erosion, thus increasing the LOC content [22,33]. Soil texture can a ect the soil aggregation processes and, therefore, influences the soil capacity to sequester organic carbon [36]. Tian et al. [37] in the alpine grassland on the Tibetan Plateau reported that soil organic carbon and total nitrogen stocks positively correlated with clay content and silt content, while they negatively related to sand content. Land use changes can indirectly a ect soil texture through the redistribution of soil by erosional processes or tillage. Revegetation on the cropland in this study could reduce the soil erosion by increasing the vegetation cover and decreasing soil disturbance, thus indirectly a ecting the content of LOC and soil available nutrients. Compared with corn treatment, revegetation did not increase the AN contents in the study area, and the corn treatment had the highest AN content in the 0 to 50 cm depth (Figure 4B). This might be due to the fertilization management in corn treatment, which applied approximately 74 kg N ha every year. Another reason for the higher AN contents under corn treatment could partially result from the short term revegetation under the revegetation land, which had limited e ects on the accumulation of AN and other soil nutrients. Besides, no significant di erences among the forage and grasslands also suggested negligible e ects of short term revegetation on the AN contents in the study area. The higher AN contents under the corn treatment were similar to the results by Zhang et al. [38] in Guizhou, China, who also reported that the AN content under fertilized and plowed cropland was higher than that under grassland and forestland. Soil AP and AK contents were not significantly di erent under most land use treatments (Figure 5), indicating that the short term land use treatments did not change the AP and AK contents in northeastern China. Similar to the changes in AN content, the negligible e ects of short term revegetation on soil AP and AK may be the primary reason for the narrow changes in AP and AK content under the five land use treatments. These results were in agreement with the findings of Zhao et al. [39] in another region of Songnen plain, who also found that the changes in AP and AK content under cropland and grassland were very limited. The SR of soil parameters was used as an indicator of the dynamics soil quality, and it could detect the management induced changes in the soil profiles of agricultural systems [15]. The increase in SR values of LOC and soil nutrient indicated the improvement of soil quality due to the accumulation of LOC and soil nutrients in the surface soil [30,40]. Land use treatments had no significant e ects on the SR values of LOC, AN, AP, and AK contents at depths of 0–10/10–20 cm and 0–10/20–30 cm except the SR of AP at the depth of 0–10/20–30 cm, suggesting that short term revegetation had limited e ects on the soil available nutrients in northeastern China. Studies in Columbia and Georgia showed that the SR values of SOC and total nitrogen were >2 under no tillage management, indicting an improvement of soil quality [29]. Peregrina et al. [41], Corral-Fernandez et al. [42], Francaviglia et al. [40], and Deng et al. [15] confirmed this finding, arguing that a high SR value (usually >2) indicated a better soil quality and contribution to agriculture sustainability. Our results showed that the SR values of LOC and soil available nutrients at the depth of 0–10/10–20 cm were mostly <2, and the SR values at the depth of 0–10/20–30 cm were mostly >2, indicating that soils under the same land use treatments had di erent soil quality. Similarly, the study by Deng et al. [15] also found that the SR values at the depth of 0–20/20–40 cm were generally higher than those at the depth of 0–5/10–20 cm found by Wang et al. [43] in the same region of the Loess Plateau. The SR values of LOC and soil nutrients in di erent soil depths in response to land use treatment were not consistent, suggesting that standard SR values of soil properties are needed in future studies to make the comparisons of soil quality under di erent management practices and di erent regions easier. Therefore, the SR values at the depth of 0–10/10–20 cm may be well suitable as a standard for evaluating significant changes in surface soils induced by management practices. In this study, three soil available nutrients including AN, AP, and AK contents and LOC were evaluated, but similar trends were not found (Figures 4 and 5). In fact, it is dicult to draw meaningful conclusions about soil quality changes when univariate indicators are used to analyze datasets involving Land 2020, 9, 10 12 of 14 many soil properties and reveal the changes within the soil environment [44]. The two indices of GMSN and SSAN were able to overcome the above weaknesses, and they were used as useful indictors of soil quality in other studies [22,26,31]. However, the results in this study showed no significant di erences of GMSN and SSAN among the five land use treatments at each soil depth except SSAN under the MLG + M treatment in the 0 to 10 cm depth (Figure 7), indicating that short term conversions of cropland to revegetation land had limited demonstrable influences on the soil available nutrients and LOC in the salt a ected region of Songnen plain. The inconclusive results suggested that a long term study is needed to examine the responses of the LOC and soil available nutrients to long term revegetation in northeastern China. 5. Conclusions The present results showed that revegetation on the cropland enhanced the LOC contents and decreased the AN contents in the 0–50 cm depth compared with the Corn treatment, and the changes in AP and AK contents were very limited after the land use conversions. The SR values in di erent soil depths in response to land uses were not consistent, suggesting that standard SR values of soil properties are needed in the future studies and that the SR values at the depth of 0–10:10–20 may be suitable as the standard considering the notable changes in surface soils induced by management practices. However, more studies are needed to examine if the SR value at the 0–10:10–20 cm is suitable in other managements or regions. The values of SR, GMSA, and SSAN were not a ected by the land use changes, indicating short term revegetation on the cropland had limited influences on the changes in soil nutrients and LOC in northeastern China. Compared with AG treatment, values of GMSA and SSAN were slightly lower than other land use treatments. These results were mainly due to the very short term (five years) revegetation because revegetation may need more time to be incorporated. Therefore, more studies are needed to assess the long term (more than 10 years) e ects of revegetation on soil properties in the Songnen grassland in the future. Although changes in soil available nutrients were given in this study, variations in soil microbial populations, which are more sensitive to changes in land uses than soil nutrients, were not mentioned. The influences of short term revegetation on soil quality need to be comprehensively assessed. In addition, we recommend that farmers in Northeast China should use revegetation to rehab grassland in areas with poor quality soils in the long run. Author Contributions: Conceptualization, P.Y., S.L., X.T., and W.L.; methodology, P.Y., X.T., and A.Z.; investigation, P.Y. and X.T.; data curation, P.Y. and A.Z.; writing, original draft preparation, P.Y.; writing, review and editing, S.L., W.L., X.T., and A.Z.; project administration, P.Y., S.L. and W.L.; funding acquisition, P.Y. and S.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Fundamental Research Funds for the Central Universities in China, Grant Numbers SWU019024 and SWU019023; the National Natural Science Foundation of China, Grant Number 41601124 and 31500446; the Excellent Young Foundation of Jilin Province, Grant Number 20190103141JH; and the University Innovation Research Group of Chongqing (Remote sensing of fragile ecological environment in southwest China). Conflicts of Interest: The authors declare no conflict of interest. References 1. Sardans, J.; Bartrons, M.; Margalef, O.; Gargallo-Garriga, A.; Janssens, I.A.; Ciais, P.; Obersteiner, M.; Sigurdsson, B.D.; Chen, H.Y.H.; Penuelas, J. Plant invasion is associated with higher plant-soil nutrient concentrations in nutrient-poor environments. Glob. Chang. Biol. 2017, 23, 1282–1291. [CrossRef] [PubMed] 2. Yu, P.J.; Liu, S.W.; Xu, Q.; Fan, G.H.; Huang, Y.X.; Zhou, D.W. 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Land – Multidisciplinary Digital Publishing Institute

Published: Jan 2, 2020

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