CHEN Jing-rui,QIN Wen-jing,CHEN Xiao-fen,CAO Wei-dong,QIAN Guo-min,LIU Jia,XU Chang-xu

1 Institute of Soil &Fertilizer and Resources &Environment,Jiangxi Academy of Agricultural Sciences,Nanchang 330200,P.R.China

2 Institute of Agricultural Resources and Regional Planning,Chinese Academy of Agricultural Sciences,Beijing 100081,P.R.China

3 Dengjiabu Original Rice Breeding Farm,Yingtan 335000,P.R.China

Abstract Green manure can be used as a substitute for chemical fertilizer without reducing rice yield.We studied the responses of soil fertility and rice yield to different combinations of Chinese milk vetch (CMV;Astragalus sinicus L.) and chemical fertilizer in a subtropical double-rice cropping system.Our goal is to reduce chemical fertilizer use and decrease environmental contamination.Compared with the recommended rate of chemical fertilizer (CF),both early-and late-rice yields in the two treatments supplied with 15 and 22.5 Mg CMV ha–1 plus 60% CF (represented as 60A and 60B,respectively) showed no significant differences while the two treatments supplied with 30 and 37.5 Mg CMV ha–1 plus 60% CF (represented as 60C and 60D,respectively) showed significantly higher values.The sustainable yield index (SYI) values in the 60C and 60D treatments with double-rice croppong system were significantly higher than those in other treatments (P<0.05).Early-rice yield showed a significant positive relationship with the Chinese milk vetch incorporation rate.The coefficients increased annually from 2009 to 2013 and then decreased in 2014.Soil organic matter increased over time by the end of the experiment in all of the treatment groups.Soil organic matter in 60A,60B and 60C showed no significant difference compared with that in CF,while soil organic matter in 60D was significantly higher than that in CF.The slopes of soil organic matter and total nitrogen over six years were the highest in 60C and 60D.The soil total nitrogen content in 60A,60B,60C and 60D was higher than that in CF,but the differences were not significant (P>0.05).Therefore,a relatively high Chinese milk vetch incorporation rate (≥30 Mg ha–1) was more effective in improving the productivity and sustainability of paddy soil.The decreased coefficients of early-rice yield and the Chinese milk vetch incorporation rate in 2014 implied that the benefits of soil fertility and rice yield created by Chinese milk vetch input may decline after five years under a continuously high rate of Chinese milk vetch incorporation.

Keywords:Chinese milk vetch,double-rice cropping system,reduced chemical fertilizer,sustainability

1.Introduction

Rice (OryzasativaL.) feeds more than 65% of the total population in China (Xin and Li 2009).China has 18.7%of the global rice growing area and produces 30% of the world’s total rice production (FAOSTAT 2013).Modern rice yields have significantly increased due,in part,to the use of chemical fertilizers (Zhanget al.2016).Chemical fertilizers play an important role in maintaining rice yields and improving grain quality (Zhanget al.2016).Inorganic nitrogen (N) fertilizer is typically applied to rice fields at a rate of 180 to 200 kg N ha–1per planting season in order to achieve optimal yield in southern China (Penget al.2002),but the N use efficiency is only approximately 30–35%(Zhu 2008).Excessive inorganic N fertilizer application wastes valuable chemicals and also reduces soil,water and atmospheric quality (Zhu and Chen 2002;Maliket al.2009;Kourgialaset al.2017).

The application of organic fertilizers sustained rice production for centuries before the introduction of inorganic fertilizers,and use of these materials has been recommended as a practical option for maintaining rice yields while lowering environmental risks and reducing production costs (Biet al.2009;Leeet al.2010).Livestock and poultry manure contain toxic antibiotics and heavy metals risks (Nicholsonet al.1999;Jiet al.2012;Zhang Fet al.2012),while leguminous green manure,through biological N2fixation,can provide an environmentally friendly and sustainable source of N to cropping systems.Leguminous green manure is a clean organic manure resource lacking the presence of antibiotics,heavy metals and other residual hazards (Gaoet al.2015).

Green manure from legumes,through biological processes,fix N2to soil,and then increase the soil N supply to subsequent crops and can partially replace inorganic N fertilizers (Aulakhet al.2000;Liet al.2015).Green manure application to soil is considered a beneficial practice in many agricultural production systems.It can increase the sustainability of cropping systems by increasing soil microbial activities (Elfstrandet al.2007;Kataokaet al.2017),soil organic matter and fertility levels (Astieret al.2006;Yanget al.2014;Xieet al.2016),and by reducing the density and population of weed species (Krishnan and Nissen 1998).Growing green manure in paddy fields during the fallow season takes full advantage of the natural resources (such as light,heat and water) and also improves rice yield and soil properties with low environmental and economic costs (Leeet al.2010;Yanget al.2014).

Chinese milk vetch (CMV;AstragalussinicusL.)cultivation has been used for more than two thousand years in China.It is a traditional winter annual green manure used for sustainable production in paddy soils in Asia (Choet al.1998;Kimet al.2008).CMV incorporated into soil can be a partial or total substitute for chemical N fertilizer for the subsequent rice season.Equivalent or higher rice yields can be obtained compared to reliance on only chemical fertilizers(Leeet al.2010;Zhuet al.2012;Zhaoet al.2015).Former studies mainly concentrated on the impacts of CMV on rice yield with short-term experiments in single or double rice cropping systems (Kimet al.2008;Leeet al.2010).CMV was treated as an extra N source in most studies (Zhang Wet al.2012;Zhuet al.2014).Therefore,the influence of continuous CMV application combined with reduced chemical fertilizer on the rice production sustainability and soil fertility in double-rice cropping systems is not well understood.Xieet al.(2016) reported that 20–40% of fertilizer N could be substituted by CMV while maintaining an equal rice yield in N-rich reddish soil.How soil fertility and rice yield respond to continuous CMV application in paddy soil with a moderate or low N content is unknown.

Double-rice cropping systems occupy a total area of approximately 6×106ha and account for >50% of the rice produced in China (Xin and Li 2009).Chinese rice yield must increase by 1.21% annually to meet the demands of the increasing population (Wu and Li 2002);therefore,sustaining the soil productivity and rice yields in double-rice cropping system is critical for food security.However,soil organic carbon in subtropical China is also showing declining trends caused by reduced inputs such as crop residues and green manure (Chenet al.2016).Also,paddy soils with high soil organic carbon levels are more sensitive to reductions in carbon input than those with lower levels of soil organic carbon (Chenet al.2016).This indicates that productivity loss is more likely to occur in high quality paddy soil when carbon inputs are reduced.Therefore,determining the most appropriate rates of green manure and chemical fertilizer application and making the best use of green manure resources are important for resource conservation and environmental protection.The aims of the present study were to:1) compare the effects of different combinations of CMV and chemical fertilizer on the sustainability of rice yield and soil fertility and 2) explore appropriate methods to maintain and/or improve rice yield and soil fertility in a subtropical double-rice cropping system.

2.Materials and methods

2.1.Site description,experimental design and management

The experiment was conducted at the Dengjiabu Original Rice Breeding Farm,Yujiang County,Jiangxi Province,China(28°12´N,116°47´E).The location has a subtropical monsoon climate.Its mean annual temperature is 17.6°C,with annual rainfall of 1 788.8 mm.The paddy soil is derived from alluvial deposits.The initial soil organic matter (SOM),total N (TN),total phosphorus,and total potassium contents of the plough layer (0–15 cm) were 27.4,1.85,0.36,and 35.2 g kg–1,respectively.Available N,P and K was 123.1,4.4 and 56.0 mg kg–1,respectively.Soil pH was 5.07 (1:1,soil:water).

The experiment was initiated in October 2008,and continued for six consecutive CMV–rice–rice cycles.Treatments were arranged in a randomized complete block design with three replicates:control (no chemical fertilizer or CMV,CK);22.5 Mg CMV ha–1(fresh weight);recommended chemical fertilizer (CF);60% CF plus 15 Mg CMV ha–1(fresh weight) (60A);60% CF plus 22.5 Mg CMV ha–1(fresh weight)(60B);60% CF plus 30 Mg CMV ha–1(fresh weight) (60C);60% CF plus 37.5 Mg CMV ha–1(fresh weight) (60D).The N content in fresh CMV was 2.6 g kg–1;therefore,the amount of N substituted by CMV in 60A,60B,60C and 60D was 39.1,58.7,78.2 and 97.8 kg ha–1,respectively.The area of each replicate was 20 m2.The recommended CF rate was:N–P2O–K2O=150–75–120 kg ha–1for early rice and 60%CF was N–P2O5–K2O=90–45–72 kg ha–1for early rice.The chemical fertilizer rate for late rice was N–P2O5–K2O=180–75–150 kg ha–1for all of the treatments except CK and CMV.Chemical nitrogen,phosphorus and potassium fertilizers were applied as urea (46% N),superphosphate (12.0%P2O5),and potassium chloride (60% K2O),respectively.Phosphorus and potassium fertilizers were applied once as basal fertilizer.40% of N was applied as a base fertilizer,and the rest was applied equally at the tillering stage and panicle initiation stage,respectively.

CMV (Yujiangdaye) was seeded at a rate of 30 kg ha–1every year in mid-October in all treatments except for the CK and CF and was harvested and incorporated into the soil 10 days before the early rice was transplanted.The weight of CMV was measured,and the CMV was incorporated into the soil as required by the experiment.Early rice(Zhongzu 7) was transplanted in late April and harvested in mid-July.Late rice (Nongxiang 98) was transplanted in late July and harvested in mid-November every year.

2.2.Sampling and analyses

After harvested the late rice,five soil samples were collected from each replicate from the plough layer (0–15 cm) each year using a soil auger.The samples were air-dried,ground and sieved through a 0.25-mm sieve to remove roots and debris prior to analysis.SOM was determined using a wet oxidation method,and TN was measured using the micro-Kjeldahl digestion method (Zhaoet al.2015).

At each harvesting time,rice plants were manually harvested at ground level from each replicate.Grain yields were obtained by direct weighing after the crop was threshed and the moisture content was determined.

2.3.Statistical analysis

The sustainable yield index (SYI) is used to measure the sustainability of a cropping system under a set of management practices (Singhet al.1990).Yield data were used for calculating the SYI as follows:

whereYaveis the mean yield,σn–1is the standard deviation,andYmaxis the maximum yield.

The agronomic efficiency of applied chemical N (AEN)was calculated from the following equation:

whereYNandY0represent the rice grain yield (kg ha–1) with or without N fertilizer,respectively.FNis the rate of applied chemical N.

All statistical analyses were performed using SPSS 13.0(SPSS for Windows,ver.13.0).Significant differences among different treatments were analyzed by repeatedmeasures ANOVA (RM-ANOVA) with treatments as the main factor and year as the repeated measure.Separation of means was analyzed by Tukey’s HSD method.Relationships between rice yield and the CMV application rate were analyzed by linear regression analyses.Linear regression analyses of soil chemical properties (SOM and TN) across years were performed to determine their trends(slopes).Lowercase letters indicate statistically significant differences atP<0.05.

3.Results

3.1.Changes in rice grain yield

Compared to the CF treatment,the 60C and 60D treatments clearly increased early-rice yield over the six-year period(Fig.1 and Table 1).The high CMV application (≥30 Mg ha–1) showed a more pronounced effect in improving the early-rice yield with an increase in experiment duration.The higher SYI values of the 60C and 60D treatments also indicated that they had higher potential to maintain sustainable rice yield production (Table 1).Although the late-rice yields of the 60C and 60D treatments in each year showed no significant differences with those of CF,the laterice yields of the 60C and 60D treatments over the six-year period were significantly higher than those of CF (P<0.05)(Fig.2 and Table 1).

The early-,late-and double-rice yields of the 60A and 60B treatments over the six-year period were not significantly different from those of CF (Table 1),but the early-rice yield of the 60A treatment was significantly lower than that of CF from 2012–2014.RM-ANOVA was applied to analyze the variance in grain yield during the six years.The results showed significant treatment and year×treatment interactions in the early-,late-,and double-rice systems (Table 1).

3.2.Agronomic efficiency of applied chemical N (AEN)

Fig.1 Continuous changes in early-rice grain yields in different treatments.CK,control;CMV,22.5 Mg Chinese milk vetch;CF,recommended chemical fertilizer;60A,60% CF plus 15 Mg CMV ha–1;60B,60% CF plus 22.5 Mg CMV ha–1;60C,60% CF plus 30 Mg CMV ha–1;60D,60% CF plus 37.5 Mg CMV ha–1.Different letters mean significant differences at 5%.Vertical bars are the standard error of the mean.

Table 1 Total rice yield (×103 kg ha–1) and sustainable yield index (SYI) of early-,late-and double-rice systems in different treatments over the six-year period (2009–2014)

Solo chemical fertilizer application resulted in a significantly lower AENvalue (21.65 kg kg–1in the early rice and 18.33 kg kg–1in the double rice) in both the early and double rice(P<0.05) (Fig.3).In both the early and double rice,the AENvalue increased with an increase in the CMV application rate,and the differences in the AENvalue among different treatments were significant in the early rice.The 60C and 60D treatments had a significantly higher AENvalue in all of the seasons,and the differences were significant in the early-and double-rice systems.

3.3.Relationship between CMV application rate and early-rice yield

Early-rice yield showed a significantly positive relationship with the CMV application rate from 2009 to 2014 (Fig.4).From 2009 to 2013,the slope increased with the duration of the experiment followed by a decrease in 2014.

3.4.Soil chemical properties

The SOM of all of the treatments in the plough layer increased after six years compared with the base value(27.40 g kg–1),while the SOM in all of the treatments except CK exhibited a significant increasing trend over time (Table 2).The highest SOM content (32.02 g kg–1)was measured in the 60D treatment and was significantly higher than those of the other treatments (P<0.05).The two highest SOM values,as well as the trends of their slopes,were observed in the 60C and 60D treatments (Table 2).

Fig.2 Continuous changes in late-rice grain yields in different treatments.CK,control;CMV,22.5 Mg Chinese milk vetch;CF,recommended chemical fertilizer;60A,60% CF plus 15 Mg CMV ha–1;60B,60% CF plus 22.5 Mg CMV ha–1;60C,60% CF plus 30 Mg CMV ha–1;60D,60% CF plus 37.5 Mg CMV ha–1.Different letters mean significant differences at 5%.Vertical bars are the standard error of the mean.

Fig.3 The mean agronomic efficiency of applied chemical N in the double-rice system over six years in different treatments.CF,recommended chemical fertilizer;60A,60% CF plus 15 Mg Chinese milk vetch (CMV) ha–1;60B,60% CF plus 22.5 Mg CMV ha–1;60C,60% CF plus 30 Mg CMV ha–1;60D,60% CF plus 37.5 Mg CMV ha–1.Different letters mean significant differences at 5%.Vertical bars are the standard error of the mean.

After six years of cultivation,the TN content in plough layer increased in all of the treatments except CK and CMV (Table 2).Before the experiment,the TN content in the plough layer was 1.85 g kg–1,which decreased to 1.71 g kg–1in CK but was maintained in CMV at the end of the experiment.The highest TN value (32.02 g kg–1) was also measured in the 60D treatment but was only significantly higher than those of CK and CMV.The TN contents in all of the treatments showed significant increasing trends except for CK and CMV,and the slope increased as the CMV rate increased.The TN content in CK exhibited a significant negative trend and had a lower TN content at the end of the experiment (Table 2).The interactions of year,treatment and year×treatment for SOM and TN were all found significant in the plough layer (Table 3).

4.Discussion

4.1.Effect of solo application of green manure on grain yield and soil properties

As a substitute for mineral fertilizer,green manure is more environmentally friendly and also has the potential to sustain grain yield and soil productivity with lower input (Changet al.2014;Gaudinet al.2014;Coombset al.2017).Therefore,the planting of green manure has received widespread interest.In 2008,the seeded area of green manure reached 437×106ha and produced 9 339 Mt of green manure in China (Yanget al.2014).Leeet al.(2010) reported that green manure could totally replace chemical fertilizer with the same or higher grain yields when the incorporation rate exceeded 10 Mg CMV ha–1in a mono-rice cropping system.Although the solo CMV application was 22.5 Mg ha–1in this study,it produced much lower grain yield than the CF in both early and late rice.This may be attributed to the differences in climate,chemical fertilizer rate and cropping system between the two experiments.However,the annual grain yield was similar to that reported for a mono-rice cropping system (Leeet al.2010).The increased SOM content and unchanged TN content in the soil indicated that even in a double-rice cropping system,solo CMV application has the potential to maintain soil productivity and sustainability with a relatively low annual grain yield.

Fig.4 Relationship between Chinese milk vetch (CMV) application amount and early-rice yield during the 2009–2014 period.＊,significant at P<0.05.＊＊,significant at P<0.01.

Table 2 Mean soil organic matter and total nitrogen in the plough layer (0–15 cm) measured in 2014 after late rice was harvested and their slopes over six years in the double-rice system (2009–2014)

Table 3 Analysis of variance (ANOVA) results of soil organic matter and total nitrogen content in the plough layer after late rice was harvested in different treatments in the double-rice system (2009–2014)

4.2.Effect of integrated green manure and mineral fertilizer on soil properties

Before mineral fertilizer was introduced,the productivity and sustainability of agricultural systems were maintained mainly by maintaining the equilibrium between N loss from crop harvest and N gain from biological N fixation(Ladha and Peoples 1995).However,this equilibrium has been disturbed in intensive cropping systems at present.Compared with the pre-chemical era,rice yield has increased significantly with the help of mineral fertilizers,but,at the same time,the whole paddy soil ecosystem has deteriorated (Zhu and Chen 2002;Maliket al.2009).

Solo application of chemical fertilizers in current intensive rice cropping has increased the problems associated with the soil and environment,as well as the maintenance of rice yield (Zhu and Chen 2002;Zenget al.2014).Green manure is considered to be one measure that can maintain soil sustainability and can also improve rice yield with a low environmental risk (Gaoet al.2015).Green manure,as a substitute for mineral fertilizer,has the potential to maintain the soil organic matter content (Elfstrandet al.2007) and increase soil N accumulation,microbial biomass and enzyme activity (Kimetuet al.2008;Dangaet al.2009;Kataokaet al.2017).Green manure with reduced chemical fertilizer promotes soil microbial development and nutrient availability in paddy fields.This combination is a promising approach in the development of more sustainable fertilization strategies (Liuet al.2009).

SOC and TN contents are the most important indexes of paddy soil fertility (Liet al.2010).Green manure combined with chemical fertilizer increased both SOM and TN contents in rice–rice or rice–wheat cropping systems within different areas (Waliaet al.2010;Nayaket al.2014;Yanget al.2014).However,green manure was applied as an extra fertilizer source in most previous studies.In the present study,the four treatments with both green manure and chemical fertilizer showed advantages in terms of SOC and TN improvement compared with CF,except for the 60A treatment.The main reason is that plant residues play a major role in forming and stabilizing macroaggregates (Skjemstadet al.1990;Cambardella and Elliott 1993),and macroaggregates of 2–5 mm contribute most to soil fertility (Yanget al.2014).Long-term experiments have demonstrated that green manure incorporation increased the number of>2 mm aggregates in the plowed soil layer and increased the content of organic carbon in all size fractions except for the 0.25–0.5 mm class,as well as the content of total nitrogen in all of the aggregate size fractions (Chenet al.2009).Other studies have shown that improved formation of macroaggregate-associated C and N occurred in longterm green manure-planted soil (Yanget al.2014).Soil aggregates of different sizes provide diverse microhabitats for microorganisms and thus influence soil enzyme activities(Muruganandamet al.2009).Macroaggregates favor N mineralization enzymes (Muruganandamet al.2009) and are more highly correlated with enzyme activity and other microbes compared to other size fractions (Miller and Dick 1995;Schutter and Dick 2002).The long-term application of green manure can change microbial communities,thereby increasing the ammonium supply and decreasing soil acidification (Gaoet al.2015).The combination of GM and mineral fertilizer improves SOC and soil TN by improving soil conditions (higher fertility,nutrient availability and microbial activity) for a crop compared with solo mineral fertilizer (Liuet al.2009;Gaoet al.2015).In addition,this combination produces higher plant biomass.Consequently,more root exudates and more stubble decompose within the soil,and the carbon and nitrogen derived from these root exudates and stubble are fixed by soil microbes.A positive cycle is created among the fertilizer,crop,microbes and soil components.Ultimately,a higher amount of carbon and nitrogen is fixed in the soil,which improves soil fertility and maintains soil productivity.

We found that SOM and TN did not increase linearly as the CMV application rate doubled or quadrupled.Three main mechanisms of SOM stabilization -chemical stabilization,physical protection and biochemical stabilization -have been proposed (Stevenson 1994;Christensen 1996).All of them directly or indirectly require the participation of microbes to form new SOM.Different kinds of microbial communities favor different ratios of C/N.A reasonable C/N ratio is the key factor in determining the formation of SOM.Winter green manure generally contains 15–20%organic matter and has a relatively high C/N ratio.Hence,excessive fresh green manure incorporation without proper fertilizer–N may have a negative impact on soil fertility.Xieet al.(2016) found that when the proportion of chemical fertilizer–N substituted by green manure increased from 20 to 80% in a double-rice cropping system,SOM and TN declined after a six-year period.However,we found that SOM and TN increased at the end of the experiment and they had similar CMV application rates.The slope trends increased as more CMV was applied (Table 2).Fresh CMV decomposed rapidly in the first month (Zhuet al.2014),and chemical fertilizer–N was reduced as more CMV was applied in order to maintain the same level of N input as in Xieet al.(2016).Therefore,other N sources (SOM-derived N and soil residue N) will be consumed when chemical fertilizer–N cannot provide sufficient N to meet the demands of CMV decomposition.Ultimately,this will lead to a decrease of the SOM and TN content.

4.3.Effects of integrated CMV and chemical fertilizer on rice yield

After CMV incorporation in the first year,neither early-nor late-rice yield showed significant differences between CF and the four other treatments with integrated fertilizers (Figs.1 and 2).This reflected the fact that soil may accumulate a high amount of chemical fertilizer after long-term rice cultivation because residual N in soil can account for more than 30% of applied fertilizer N (Liet al.2015).Therefore,the impact of different amounts of CMV application on rice yield might be masked by the high residual N content in the soil in the first year.

Early-rice yield increased as the CMV application rate increased when chemical fertilizer application was the same (Fig.1),and regression analysis also found a linear,positive relationship between early-rice yield and the CMV amount (Fig.4).This means that the higher early-rice yield compared with CF in 60C and 60D partially attributed to the higher total nitrogen (N derived from both chemical fertilizer and CMV) application.On the one hand,CMV decomposition in the first month released more than 80% of the total nitrogen and other nutrients (Zhuet al.2014),which might be more than the reduced fertilizer could provide.On the other hand,CMV combined with reduced chemical fertilizer treatments retained more chemical nitrogen in the plough soil layer than solo chemical fertilizer treatments(Wanget al.2014).These nutrients provided a solid foundation for early rice to achieve a high yield.Isotopic research confirmed that CMV application enhanced rice nitrogen uptake,especially the nitrogen derived from CMV decomposition (Wanget al.2014).In addition,Wanget al.(2014) reported that chemical NUE was increased when CMV was applied,which was similar to our results (Fig.3).After the early rice was harvested,although the nutrients derived from CMV were still released,the effect on late rice may have been masked by the heavy chemical fertilizer application (N=180 kg ha–1) used in this study.

The increased slope during 2009–2013 and the decreased slope in 2014 implied a strong potential for early-rice yield when more CMV was applied in the first five years after CMV incorporation.An asymptotic relationship was also found between C input and the SOC content (Sixet al.2002).Therefore,we inferred that continuous CMV application in this study may have reached a transient balance between C input and SOC content after five years.In addition,the CMV application rate had a higher impact on early-rice yield during the first five years of CMV incorporation,whereas the impact subsequently decreased.Further study will be required to verify the accuracy of this interpretation.

CMV applied with chemical fertilizer treatments retained more N residue compared with solo chemical fertilizer treatment after two consecutive rice seasons (Zhuet al.2014).After long-term consecutive green manure application,nitrogen and other nutrients may accumulate in the soil.The resulting improved soil nutrient supply capacity will help guarantee a stable and sustainable rice yield.This can partially explain the increased value of SYI as the CMV application rate increased.

5.Conclusion

The present study demonstrated that continuous incorporation of Chinese milk vetch with 60% of the recommended rate of chemical fertilizer (decreased only in the early-rice season) could maintain rice yield and soil fertility in a double-rice cropping system.The impact of green manure application on rice yield was significant in both the early and late rice over a six-year period.Early-rice yield showed a significantly positive linear relationship with the green manure rate.A strong potential for increasing early-rice yield was observed for five years,after which the potential decreased.We inferred that although a high CMV application rate (≥30 Mg ha–1) was more effective in improving the productivity and sustainability of paddy soil,long-term consecutive green manure application may lower the sensitivity of rice yield to organic carbon input.

Acknowledgements

This research was funded by the National Key Research and Development Program of China (2016YFD0800500 and 2016YFD0200800),the National Natural Science Foundation of China (31860592),the earmarked fund for China Agriculture Research System (CARS-22-Z-06),the Special Scientific Research Fund of Agricultural Public Welfare Profession of China (201503123 -07),and the Innovation Fund of Jiangxi Academy of Agricultural Sciences,China (2015CBS007 and 20182CBS002).