Effects of different continuous fertilizer managements on soil total nitrogen stocks in China:A meta-analysis

Pengfei DANG,Congfeng LI,Tiantian HUANG,Chen LU,Yajun LI,Xiaoliang QINand Kadambot H.M.SIDDIQUE

1College of Agronomy,Northwest A&F University,Yangling 712100(China)

2Institute of Crop Science,Chinese Academy of Agricultural Sciences/Key Laboratory of Crop Physiology and Ecology,Ministry of Agriculture,Beijing 100081(China)

3Institute of Agriculture and School of Agriculture&Environment,The University of Western Australia,PerthWA 6001(Australia)

ABSTRACT Soil total nitrogen is critical for crop productivity and related to agricultural managements.However,the effects of different fertilizer applications on soil total nitrogen storage are not well understood.To quantify soil total nitrogen storage under different fertilizer management practices and explore the effects of climate,soil texture,experimental duration,and cropping system on soil total nitrogen storage in China,we conducted a meta-analysis of 67 fertilizer management strategies from experiments conducted over a period of at least three years.This meta-analysis included 854 observations of changes in soil total nitrogen stock(TNS)under no fertilizer application(control,CK),chemical fertilization with nitrogen,phosphorus,and potassium(CF),CFplus straw retention(CFS),and CFplus manure addition(CFM)relative to initial soil TNS.The CFM and CFS treatments increased soil TNS,and the CFM treatments increased soil C/N ratio the most.The longer the experimental duration,the greater the increase in soil TNS in the CF,CFS,and CFM treatments.Soil texture and crop type significantly affected the changes in soil TNS.The experimental duration,initial soil TNS,soil C/N ratio,and cropping system had significant linear correlations with the change in soil TNS.Temperature and precipitation were not correlated with soil TNS.Results of random forest modeling indicated that the most important factor affecting changes in soil TNS was experimental duration(positive correlation),followed by initial soil TNS(negative correlation).The CFM treatments had the largest increase in soil TNS under various conditions.We recommend promoting CFM to improve soil fertility in farmlands globally.

Key Words:chemical fertilizer,climate change,cropping systems,manure,soil fertility,straw retention

INTRODUCTION

Soil total nitrogen provides crops with sufficient nitrogen and promotes crop growth(Denget al.,2020)by enhancing carbon uptake and stimulating crop photosynthesis(Voset al.,2005).Soil organic nitrogen,which accounts for approximately 95% of soil total nitrogen(Yeet al.,2018),can be mineralized and subsequently absorbed by plants(Mooshammeret al.,2014).Soil nitrogen and carbon contents are important indicators of soil quality.It is well known that soil total nitrogen content is positively correlated with soil organic carbon(SOC)content,and both are mutually involved in global carbon and nitrogen cycles(Gallowayet al.,2004).Soil nitrogen directly affects SOC storage in terrestrial ecosystems(Tianet al.,2006;Denget al.,2020).High nitrogen input could increase SOC content in farmland soils(Guoet al.,2012).Many studies have focused on SOC storage(Tianet al.,2015;Hanet al.,2016;Berhaneet al.,2020);however,few studies have been conducted on changes in soil total nitrogen stock(TNS)in the field.Understanding soil TNS is essential for reducing greenhouse gas emissions and nitrogen leaching,which is important for soil managements and biogeochemical cycling.

Fertilizer managements are crucial for increasing crop production and improving soil fertility and soil total nitrogen content(Gallowayet al.,2004;Wuet al.,2010;Yanget al.,2015;Caiet al.,2019;Yuet al.,2020).Many fertilizer regimes are used in agricultural production in China,including chemical fertilization with nitrogen,phosphorus,and potassium(CF),CFplus straw retention(CFS),and CFplus manure addition(CFM).Manure or straw addition improves soil aggregation and nitrogen retention in aggregates of different particle sizes(Sodhiet al.,2009).Studies have shown that CFS and CFM significantly increase soil total nitrogen content and storage(Yanget al.,2015;Wanget al.,2018;Qaswaret al.,2020;Suet al.,2020;Yuet al.,2020).However,the changes in soil total nitrogen content and storage with CFor no fertilizer application(control(CK),where nitrogen inputs include crop residues,root exudates,and atmospheric nitrogen deposition)were contradictory.Some studies have indicated that long-term CFhad no effect on soil TNS in the North China Plain(Renet al.,2015;Gaiet al.,2018),subtropical China(Tonget al.,2009),and South China(Caiet al.,2019;Maet al.,2019);however,other studies reported a significant increase in soil TNS in the North China Plain(Niuet al.,2011;Gaoet al.,2015)and South China(Nieet al.,2007;Qaswaret al.,2020),but a significant decrease in TNS in Central China(Suet al.,2020).Under CK,contradictory results in soil TNS relative to the initial TNS were also reported:no changes in the North China Plain by Heet al.(2015)and Renet al.(2015)and in South China by Tonget al.(2009),but significant increases in the North China Plain by Niuet al.(2011)and Gaoet al.(2015)and in South China by Nieet al.(2007),Tonget al.(2009),and Qaswaret al.(2020),or decreases in South China by Tonget al.(2009)and Maet al.(2019)and in Central China(Suet al.,2020).

Cropping system,soil texture,and climatic conditions significantly affect soil total nitrogen content(Gallowayet al.,2004;Bassoet al.,2016;Douet al.,2018).Temperature and precipitation can indirectly change nitrogen dynamics(Schimelet al.,1996).Soil nitrogen mineralization and nitrification rates initially increase and then decrease with increasing temperature and precipitation(Zhou and Ouyang,2001).Soils with different texture have different characteristics;for example,loam soils have good drainage,but relatively poor nutrient content,clay soils have good soil nutrient content(Yuet al.,2018),and loam soils exhibit higher nitrogen leaching(Karimiet al.,2017)and denitrification(Yuet al.,2019).Single,double,and triple cropping systems are the principal cropping systems in China(Berhaneet al.,2020),and they differ in root biomass and exudates,the two factors that affect the changes in soil TNS(Poudelet al.,2001;Huanget al.,2012).Under long-term agricultural practices,the processes of nitrogen supply and transformation are complicated by the interactive effects of soil properties,climatic conditions,fertilizer type,and cropping system(Gallowayet al.,2004;Yanet al.,2012;Bassoet al.,2016;Douet al.,2018).The long-term effects of different fertilizer managements on soil TNS remain unclear(Tonget al.,2009;Caiet al.,2019;Qaswaret al.,2020;Suet al.,2020).A few studies have investigated soil total nitrogen storage in croplands,but information on other factors,such as experimental duration,soil texture,cropping system,temperature,precipitation,and initial soil TNS levels,is needed to explain the effects of different fertilizer management practices on certain soil nitrogen pools.

Meta-analyses synthesize data from multiple studies to provide stronger evidence than individual studies alone(Huanget al.,2012;Berhaneet al.,2020).Soil TNS has been found to be affected by different environmental variables in various studies(Poudelet al.,2001;Huanget al.,2012;Heet al.,2015;Renet al.,2015).We used meta-analysis to quantify the effects of long-term different fertilizer management practices on soil TNS in relation to experimental duration,soil texture,cropping system,temperature,and precipitation.We hypothesized that:i)CFM and CFS increase soil TNS more than CFand CK,ii)experimental duration,soil texture,and cropping system significantly affect soil TNS,and iii)a relationship exists between changes in soil TNS and initial soil TNS.

DATA COLLECTION AND DATA ANALYSIS

We searched Google Scholar(http://scholar.google.com/)and China National Knowledge Infrastructure(http://www.cnki.net/)for articles using key words related to soil total nitrogen,fertilizer types(chemical fertilizer or manure or straw),and long-term experiments.To minimize bias and compile a representative dataset,the studies selected had to fit the following criteria:i)initial and final soil total nitrogen contents or stock levels were reported with known years;ii)they were conducted in the field;iii)depth of soil sampling was 0—20 cm;iv)experimental duration was at least three years;and v)they included at least one of the treatments CK,CF,CFS,and CFM.Sixty-seven published papers were selected and used in this meta-analysis(Table SI,see Supplementary Material for Table SI).

The data,number of observations,experimental site,soil total nitrogen content or stock,soil bulk density,SOC,experimental duration,cropping system,soil texture,temperature,precipitation,and fertilization management,were obtained from the text and tables in the publications.Data presented in figures or charts were extracted using Engauge Digitizer 11.3(Free Software Foundation,Inc.,Boston,USA).Annual cropping systems were categorized as single(harvested once per year),double(harvested twice per year),or triple(harvested three times per year)cropping systems.Detailed information on the cropping systems used in this meta-analysis is presented in Table SI.The experimental durations were categorized into＜10,11—20,and＞20 years(Hanet al.,2016).Soil texture was categorized into three types,loam,clay loam,and clay.Mean annual temperature was categorized as＜8,8—16,and＞16°C.Mean annual precipitation was categorized as＜700,700—1400,and＞1 400 mm.

Soil TNS(Mg ha−1)was calculated as follows:

where TNC is the soil total nitrogen content(g kg−1),BD is the soil bulk density(g cm−3),His the measured soil depth(cm),and 10 is for adjusting the unit of TNS to Mg ha−1.If soil BD was not measured,we calculated the BD value using Eq.2 for upland-paddy and paddy soils(Panet al.,2021)and Eq.3 for upland soils(Berhaneet al.,2020):

We used the natural logarithm of the response ratio(RR),termed as effect size(ES),to quantify the effects of fertilizer management treatments on soil TNS:

whereXfandXiare the mean soil TNS(Mg ha−1)at the final and initial stages of each treatment,respectively.To better explain the results,ES was converted to a percentage to evaluate the change in ES(E,%)as follows:

According to Eqs.4 and 5,Eequals to the soil TNS change(%)at the final stage relative to the initial stage in a treatment;positive values indicate an increase in soil TNS,and negative values indicate a decrease.

The TNS sequestration rate(Mg ha−1year−1)was calculated using the following equation:

wheretis the number of experimental years.

We used a fixed-effect model with 9 999 iterations in MetaWin 2.1 to evaluate the mean ES(Wanget al.,2017).If the 95% confidential intervals(CIs)for the mean ES did not overlap zero,then the final soil TNS significantly increased(＞0)or decreased(＜0)compared to the initial soil TNS(P＜0.05).To determine the relationship between changes in SOC and soil TNS in long-term experiments,the percentage changes in SOC at the final stage relative to the initial stage under CK,CF,CFS,and CFM were calculated.Meta-analysis is assumed to be independent(Gurevitch and Hedges,1999;Berhaneet al.,2020);therefore,to reduce the dependency of observations,for long-term studies containing duplicate results in different years for the same field plots,we included only the latest sampling date in the analysis(except when testing experimental duration effect and the relationship between TNS and SOC stocks)(Berhaneet al.,2020).

The SPSS software(SPSS 22.0,IBM,Armonk,USA)was used for all statistical analyses.Origin 9.0(OriginLab Corp.,Northampton,USA)was used to create the graphs.One-way analysis of variance was used to evaluate the initial soil TNS under different soil texture and cropping systems.When a relationship was established,we performed data quality control and removed any outliers(values that did not fit the normal distribution).The relationship between soil TNS and SOC stocks was evaluated using an exponential regression analysis.The relationships between changes in soil TNS and environmental variables were evaluated using partial correlations.Random forest modeling was conducted in R(R Development Core Team,2012)to determine important factors in the responses to changes in soil TNS(see Renet al.(2019)for details).

RESULTS

Changes in soil TNS under different fertilization managements in relation to experimental duration

Relative to the initial TNS in soil,soil TNS after longterm fertilization managements significantly increased by 6.59%,25.54%,35.26%,and 47.51% on average in the CK,CF,CFS,and CFM treatments,respectively(Fig.1a).The CFM and CFS treatments increased soil TNS for all three experimental durations.However,the CFtreatments had no effect on soil TNS for the experimental duration of＜10 years,but increased it for the experimental durations of 10—20 years and＞20 years.In the CK treatments,soil TNS decreased for the experimental duration of＜20 years,but increased for the experimental duration of＞20 years.Soil TNS sequestration rates were positively correlated with experimental duration in the CK treatments,but not in the CF,CFS,and CFM treatments(Fig.S1,see Supplementary Material for Fig.S1).

Climate effect on changes in soil TNS

Compared to the initial TNS in soil,soil TNS after long-term fertilization managements significantly increased in all treatments under all temperature categories,except that in the CK treatments at＞16°C with almost no change(CFM＞CFS＞CF＞CK)(Fig.1c).Similarly,soil TNS significantly increased in the CFM,CFS,and CFtreatments(CFM＞CFS＞CF)under all precipitation categories,but did not change in the CK treatments at the precipitation levels of 700—1400 and＞1400 mm(Fig.1d).

Fig.1 Percentage changes in soil total nitrogen stock(TNS)relative to the initial TNS in soil,calculated using effect size value based on response ratio as shown in Eqs.4 and 5,under long-term different fertilization managements,no fertilization(control,CK),chemical fertilization with nitrogen,phosphorus,and potassium(CF),CFplus straw retention(CFS),and CFplus addition of manure(CFM)(a)and for different experimental durations(b),different annual average temperature(AAT)categories(c),and different annual average precipitation(AAP)categories(d)under long-term different fertilization managements.Values are means with standard deviations shown by horizontal bars.

Soil texture effect on changes in soil TNS

Compared to the initial TNS in soil,TNS in clay soils after long-term fertilization managements significantly increased in the CF,CFS,and CFM treatments,but slightly decreased in the CK treatments(Fig.2a),with the percentage changes ranked as CFM(28.88%)＞CFS(7.72%)＞CF(6.60%)＞CK(−2.17%).In clay loam soils,TNS significantly increased in all treatments in the order of CFM(by 44.37%)＞CFS(by 43.23%)＞CF(by 35.31%)＞CK(by 23.17%).Similar to clay soils,TNS in loam soils also significantly increased in the CF,CFS,and CFM treatments,but slightly decreased in the CK treatment,with the percentage changes ranked as CFM(76.05%)＞CFS(36.31%)＞CF(22.67%)＞CK(−0.34%).In general,the increases in TNS in the CF,CFS,and CFM treatments were the lowest in clay soils and the highest in the loam soils.Clay soils had the highest initial TNS,and loam soils had the lowest initial TNS(Fig.3a).

Fig.2 Percentage changes in soil total nitrogen stock(TNS)relative to the initial TNS in soil,calculated using effect size value based on response ratio as shown in Eqs.4 and 5,for different soil types(a)and different cropping systems(b)under long-term different fertilization managements,no fertilization(control,CK),chemical fertilization with nitrogen,phosphorus,and potassium(CF),CFplus straw retention(CFS),and CFplus addition of manure(CFM).Values are means with standard deviations shown by horizontal bars.

Cropping system effect on changes in soil TNS

Compared to the initial TNS in soil,soil TNS after long-term fertilization managements significantly increased in all treatments in the order of CFM(by 50.98%)＞CF(by 41.05%)＞CFS(by 23.10%)＞CK(by 9.10%)under single cropping systems and CFM(by 58.81%)＞CFS(by 29.04%)＞CF(by 25.21%)＞CK(by 5.51%)under double cropping systems(Fig.2b).Under triple cropping systems,soil TNS increased in the all treatments,but not significantly in the CK and CFtreatments(no data for CFS),in the order of CFM(by 18.21%)＞CF(by 7.93%)＞CK(by 5.48%).Initial soil TNS was the highest under triple cropping systems and did not differ between single and double cropping systems(Fig.3b).The changes in soil TNS varied with crop type under different cropping systems,increasing the most under single maize cropping systems and decreasing the most under single wheat cropping systems(Table SII,see Supplementary Material for Table SII).Apart from wheat-maize cropping systems,which showed significant changes in soil TNS in all the treatments,the other cropping systems did not affect soil TNS in the CK treatments.The CF,CFS,and CFM treatments significantly increased soil TNS,with the increases being more in the CFM treatments than in the other treatments under all cropping systems(Fig.4a).Moreover,soil TNS increased in maize,wheat-maize,and rice-rice cropping systems with increasing experimental duration(Fig.4b).

Fig.3 Initial soil total nitrogen stocks(TNS)in top 20 cm layer for different soil types(a)and different cropping systems(b).Horizontal lines within boxes indicate medians.Vertical bars indicate minimum and maximum values,and circles denote outliers.Different letters indicate significant differences at P＜0.05.

Fig.4 Percentage changes in soil total nitrogen stock(TNS)relative to initial TNS in soil,calculated using effect size value based on response ratio as shown in Eqs.4 and 5,for different cropping systems(a)under long-term different fertilization managements,no fertilization(control,CK),chemical fertilization with nitrogen,phosphorus,and potassium(CF),CFplus straw retention(CFS),and CFplus addition of manure(CFM),and different experimental durations in different cropping systems(b).R-R-G=rice-rice-green manure cropping systems;R-R=rice-rice cropping systems;R-W=rice-wheat cropping systems;W-M=wheat-maize cropping systems;M=maize cropping systems;W=wheat cropping systems.Values are means with standard deviations shown by horizontal bars.

Changes in SOC stocks and soil TNS

For all treatments,soil TNS and SOC stocks had a significant(P＜0.001)positive correlation(Fig.5a),and there was a linear correlation between their percentage changes after long-term fertilization managements(Fig.5b).Changes in SOC stocks were faster compared to those in soil TNS(Fig.5b),leading to the increased C/N ratio(Table SIII,see Supplementary Material for Table SIII).

Fig.5 Relationships between soil total nitrogen stock(TNS)and soil organic carbon(SOC)stock(a)and between percentage change in soil TNS and percentage change in SOC stock relative to their initial values(b)after long-term fertilization managements.The asterisks***indicate significant differences at P＜0.001.

Multiple factors driving the changes in soil TNS

Several factors in the random forest model explained 56.58%of the variance in soil TNS after long-term fertilization managements,with experimental duration and initial soil TNS being the most important factors,followed by soil C/N ratio,AAP,AAT,and cropping system(Fig.6).Changes in soil TNS had a significant positive correlation with experimental duration and a significant negative correlation with initial soil TNS(Fig.7).

Fig.6 Relative importance,expressed as the mean squared error(MSE)increase,of independent variables for changes in soil total nitrogen stock(TNS)after long-term fertilization managements as determined by the random forest model.CS=cropping system;AAT=average annual temperature;AAP=average annual precipitation;C/N=soil C/N ratio;Ini TNS=initial TNS in soil;ED=experimental duration.

Fig.7 Partial correlations between changes in soil total nitrogen stock(TNS)after long-term fertilization managements and environmental variables.Arrow thickness is proportional to the magnitude of the partial correlation coefficient,and red and blue arrows indicate positive and negative relationships,respectively.AAP=average annual precipitation;AAT=average annual temperature;Ini TNS=initial TNS in soil;C/N=soil C/N ratio.

DISCUSSION

Changes in soil TNS under different fertilizer managements in relation to experimental duration

Total nitrogen is the core index of soil quality(Dalalet al.,2011),and substantial nitrogen sequestration is required to improve or maintain current soil TNS across all agroecosystems(Denget al.,2020).We found that CFM and CFS changed soil TNS more compared to CF,with the highest increase detected under CFM(Fig.1a,b),which is consistent with other studies(Haoet al.,2008;Zhanget al.,2009;Gaoet al.,2015;Liet al.,2018).It is likely that manure or straw addition improves soil structure and increases the proportion of large macroaggregates,indirectly improving soil nitrogen storage capacity and TNS(Sodhiet al.,2009).Some studies have reported that excessive inorganic nitrogen fertilizer is not conducive to soil structure and reduces nitrogen retention(Hanet al.,2017;Rahman and Zhang,2018).Under CF,soil TNS did not differ with inorganic nitrogen fertilizer amounts;however,under CFM and CFS,lower inorganic nitrogen fertilizer amounts increased soil TNS more compared to higher nitrogen fertilizer amounts,indicating that the amount of nitrogen fertilizer applied can be reduced under CFM and CFS(Fig.S2,see Supplementary Material for Fig.S2).The above results also suggest that CFM is the best fertilizer management practice in North China.

Soil nutrients can gradually change during long-term fertilization and cultivation practices(Huanget al.,2012;Berhaneet al.,2020;Jianet al.,2020).The experimental duration was the most significant factor affecting changes in soil TNS with fertilizer treatments(Fig.5),and its extension increased soil TNS under various fertilizer treatments.The application of CFS or CFM increases soil TNS(Gaoet al.,2015;Yanget al.,2015;Gaiet al.,2018;Berhaneet al.,2020;Yuet al.,2020).The present meta-analysis revealed that changes in soil TNS did not differ between the CFM and CFS treatments in studies＜10 years,but were more in the CFM treatments than in the CFS treatments in studies＞10 years.Other studies have reported no significant differences between CFS and CFM(Wuet al.,2017;Liet al.,2018;Eet al.,2019),possibly reflecting the short-term nature of the studies.The CFtreatments did not significantly change soil TNS in studies＜10 years,but significantly increased soil TNS in studies＞10 years.The different results may be due to differences in the experimental duration(Tonget al.,2009;Donget al.,2012;Gaoet al.,2015;Yonget al.,2018;Qaswaret al.,2020).Our analysis showed that long-term fertilization of＞20 years for all fertilizer types increased soil nitrogen pools.However,some studies have indicated that soil TNS in North China failed to reach equilibrium after 20—30 years of field experiments(Gaoet al.,2015;Gaiet al.,2018).Very few studies span more than 30 years(Table SI);changes in soil TNS after more than 30 years should be explored to define the upper limit of soil TNS,which could benefit the soil management.Furthermore,compared with the initial values,soil TNS significantly decreased under no fertilization(CK)in studies of 1—20 years,but slightly increased in studies＞20 years.The slight increase in soil TNS in studies over 20 years may be due to the significant increases in soil total nitrogen and SOC contents caused by atmospheric nitrogen deposition in the past decades(Maet al.,2020),root absorption of nutrients from deep soil required for crop growth(Kell,2011),and increased nitrogen input in topsoil from deep soil by crop stubble returning(Tonget al.,2009).

Responses of soil TNS to fertilization in soils withdifferent texture

Soil texture,which is closely related to other soil physical properties and soil chemical properties,affects soil total nitrogen cycling(Batjes,2000;Haydenet al.,2010;Pelsteret al.,2012).Changes in soil TNS were significantly different among the three types of soil texture(Table SII).Aeration can change soil mineralization rate(Rasiah and Kay,1998);in particular,soil organic matter mineralization can increase after flooding events(Jiaet al.,2017).Loam soils,owing to their large pores and good aeration(Chaudhari and Somawanshi,2004),have the highest mineralization rate,while clay soils have the lowest(Tianet al.,2016).Loam soils have the lowest sorption capacity,leading to large nutrient losses during flooding(Chaudhari and Somawanshi,2004),groundwater pollution,and the lowest initial soil TNS.A high proportion of clay protects nitrogen against microbial degradation and improves the sorption capacity for nitrogen(Hassink,1997).Clay soils had the highest sorption capacity,nutrient levels,and initial TNS,and thus they experienced the smallest change in TNS(Fig.6).The CF,CFS,and CFM treatments affected soil TNS differently for each soil texture type;more significant changes were observed under CFM and CFS than under CF,which could be due to the improved soil structure,increased soil aggregation,and reduced nitrogen loss(Heet al.,2015;Iqbalet al.,2018).The CFM and CFS treatments also had greater soil nitrogen inputs and contributed to greater soil TNS,compared to the CF treatments.

Responses of soil TNS to different cropping systems and crop types

Cropping systems play a critical role in soil TNS because they affect the balance between nitrogen input through litter or nitrogen addition and nitrogen loss through decomposition(Wittet al.,2000;Hubbardet al.,2013).The initial soil TNS had a significant negative relationship with changes in soil TNS(Fig.6).Triple cropping systems had the lowest average soil TNS,possibly because they had the highest average initial soil TNS.Single cropping systems had the lowest average initial soil TNS(Fig.2d).Multi-season cropping systems(e.g.,rice-rice-rape)must be under sufficient temperature and precipitation conditions(Huanget al.,2012);they produce more crop residues than single cropping systems(e.g.,wheat),which may increase initial soil TNS.A high proportion of clay can protect nitrogen from microbial degradation and prevent gaseous nitrogen loss(Hassink,1997).Triple cropping systems are implemented in soils with high clay content,whereas single cropping systems are applied in soils with low clay content;therefore,triple cropping systems had higher capacity to fix soil nitrogen.No significant differences in initial soil TNS were observed between single and double cropping systems(Fig.2d),which could be due to geographical differences,as the soils in Northeast China have a high clay content and are very fertile(Wen and Liang,2001).

Crop types in the various cropping systems could reflect the differences in nitrogen output and directly impact soil nitrogen stocks.Crop types had a significant effect on the changes in soil TNS(Table SII).For example,maize cropping systems had lower soil TNS than wheat cropping systems under different fertilizer management strategies(Fig.3a),which could be due to the higher biomass production in maize cropping systems.Moreover,maize grows at higher temperatures,resulting in a higher mineralization rate,compared wheat(Kanet al.,2020),which possibly contributes to the increased nitrogen output rate and the reduced nitrogen sequestration in maize cropping systems.However,not all crops that produce a high biomass result in small changes in soil TN.Maize-wheat systems that produce more biomass had a larger change in soil TNS than rice-wheat cropping systems,which may be due to the effects of soil texture,precipitation,nitrogen input,and initial soil TNS(Table SII).

Relationship between changes in SOC stock and soil TNS

Soil total nitrogen and SOC are generally associated with soil quality and sustainability(Wanget al.,2020).Many studies have focused on SOC storage and sequestration(Tianet al.,2015;Hanet al.,2016;Berhaneet al.,2020),but whether the change in soil total nitrogen equals the change in SOC under long-term fertilizer managements remains unanswered.Our results indicate a significant correlation between soil TNS and SOC stock although soil TNS increased slower than SOC stock under long-term fertilizer managements(Fig 3c,d).Moreover,some long-term fertilizer experiments have revealed that the addition of straw and manure significantly increases soil C/N ratio(Wanget al.,2014;Yonget al.,2018).Our results also indicated that soil C/N ratio increased under CF,CFS,and CFM(Table SIII),which is because the high C/N ratio of crop residues,such as roots and leaf litter(Nicolardotet al.,2001;Monfortiet al.,2015),would provide more carbon than nitrogen into soil under all fertilizer management options.The increases in soil C/N ratio were more under CFS and CFM than under CF(Table SIII),which is likely due to the higher C/N ratio in straw and manure than in soil(Wanget al.,2014);that is,the CFS and CFM treatments contributed more carbon than nitrogen into the field under long-term fertilizer managements.The nitrification rate can be predicted using the soil C/N ratio in global climate models(Panet al.,2021).High soil C/N ratio may promote nitrogen fixation and reduce nitrification(Bengtssonet al.,2003).

CONCLUSIONS

Experimental duration,initial soil TNS,soil texture,cropping system,and crop type significantly affected the changes in soil TNS after long-term different fertilization managements.Experimental duration was the most important factor affecting changes in soil TNS,followed by initial soil TNS.Changes in soil TNS after long-term fertilization managements were positively correlated with experimental duration,but negatively correlated with the initial TNS in soil.Soil TNS increased under all fertilization management practices(including no fertilization)in long-term experiments in China,with the increases being the most under CFM,followed by CFS and CF.Therefore,CFM is the recommended agricultural practice for improving soil TNS.

CONTRIBUTION OFAUTHORS

The first two authors contributed equally to this work.

ACKNOWLEDGEMENT

Financial support was provided by China Agriculture Research System of MOFand MARA(No.CARS-02-12)and the National Natural Science Foundation of China(No.31701384).

SUPPLEMENTARY MATERIAL

Supplementary material for this article can be found in the online version.