Stability Of Soil Organic Carbon And Nitrogen Under Long-Term Fertilization In Two Cropland Soils Of China

Long-term fertilization has been recognized as a key practice controlling soil organic carbon(SOC)and crop yields at various scales.Soil aggregation plays a critical role in the maintenance of soil structure,as well as in its productivity.Fertilization influences soil aggregation,especially by regulating SOC and total nitrogen(TN)contents in aggregate fractions.Moreover,the mineralization of SOC is an important process in the maintenance of soil fertility and overall productivity.Apart from crop yield increment and soil fertility improvement,sequestration of soil carbon to mitigate climate change is one of the prime objectives of long-term fertilization.However,the mechanisms regulating stability of sequestered SOC under long-term fertilization remained largely unknown.Specifically,how soil aggregation,chemical composition and biological stability of SOC respond to long-term mineral and organic fertilization has been least explored and hence require further research.In addition,the commonly used physical,chemical and biological fractionation techniques for the isolation of stable SOC fractions have not been comprehensively addressed.Therefore,the overall objectives of this research work were to evaluate the stability of SOC by using physical(aggregate and particle size)fractionation,biological incubation and 13C nuclear magnetic resonance(NMR)analysis in two major soils of China.There is a general argument that,long-term nutrient addition by chemical fertilizers and organic manures enhance SOC sequestration by enhancing soil aggregation.To evaluate the efficiency of three fertilizer regimes(unfertilized control-CK-,mineral fertilization-NPK-and manure combined with NPK-NPKM)on soil aggregate stability,aggregate-associated organic carbon and total nitrogen sequestration and mineralization of SOC the soil samples from 0-20 cm depth were collected from a red soil long-term fertilization experiment in Qiyang of Hunan province,and analysed for size distribution ranging(>250 ?m,250-53 ?m and<53 ?m sizes),SOC and TN contents,as well as for mineralization of bulk and aggregate associated-SOC.Results revealed that,Both NPK and NPKM fertilizations significantly enhanced SOC and TN contents in bulk soil and its constituent aggregates of>250 ?m,250-53 ?m and<53 ?m sizes,as compared to CK.Long-term NPK and NPKM increased SOC and TN stock in bulk soil by 61 and 119%,and by 26 and 143%,respectively,as compared to CK.Similarly,higher values of SOC and TN stock in all aggregate fractions was observed with the application of NPKM.Application of NPK and NPKM for 26 years significantly increased aggregate stability,which was positively correlated with total SOC contents in terms of mean weight diameter(MWD)(R2=0.941,p<0.0001)and geometric mean diameter(GMD)(R2=0.928,p<0.0001).Irrespective of treatments,higher cumulative C-mineralization was observed for macro-aggregates(>250 ?m size)followed by 250-53 ?m and<53 ?m size aggregates.However,manure combined with mineral fertilizer significantly declined the proportion of SOC mineralized(%of total SOC)from the bulk soil and different aggregate fractions which suggested NPKM treatment as the best strategy not only to improve soil aggregation but also it could sequester highest SOC and mitigate its losses.Physical fractionation of SOC provides a useful tool to assess the effects of long-term management practices on C sequestration.To assess the effects of long-term fertilization(both organic and inorganic)on variously protected SOC fractions,SOC associated with different fractions,and their relationships with crop productivity soil samples(0-20 cm)were collected from a typical black soil(Mollisol)located in Gongzhuling,Jilin province,Northeast China.The SOC fractions viz.?(unprotected fractions-cPOC and fPOC,physically protected fraction-iPOC,and biochemically protected fractions-s+c f and s+c_m)were isolated using physical fractionation approach.Results revealed that,manure treatments(1.5MNPK and MNPK)led to the largest increase in the SOC content of bulk soil and the isolated SOC fractions.The highest increase in SOC under the applied treatments was observed in physically protected iPOC fraction which revealed the physical protection of OC as the main mechanism of SOC sequestration in the black soil here studied.Regression analysis further suggested iPOC and s+c_f as the most sensitive fractions for change in crop yields.Similar to other parameters,enhanced maize yields(grain and straw biomass)further manifested the manure addition as the most influential management strategy(as revealed by heat map analysis)in order to improve SOC sequestration,soil fertility and crop production.Stability of soil organic carbon has recently gained much attention of the scientific community,however,mechanisms regulating SOC stability has not been assessed comprehensively.Recent literature suggests that combining multiple techniques results in comprehensive evaluation of SOC stability.This approach was performed in the last study of the present research work to evaluate SOC stability in terms of SOC mineralization(proportion of total SOC),structural composition and SOC stability indices based on 13C NMR analysis of bulk soil and different SOC fractions(cPOC,fPOC,iPOC,and MOC).Highest cumulative CO2-C mineralization was noted in cPOC fraction,whereas,lowest cumulative CO2-C mineralization was recorded in MOC fraction.Results revealed that the longterm addition of manure combined with mineral fertilization had considerable potential in having less proportional(to its total SOC)SOC decomposition.The carbonyl-C was the dominant proportion of SOC functional groups and hence the most sequestered form of SOC in the soil studied.Moreover,the SOC stability indices viz.,aromaticity index(AI),hydrophobicity index(HI)and alkyl-C-O-alkyl-C ratio were considerably higher in manure treated plots,suggesting the accumulation of more recalcitrant C compounds in the soil.Further,the percent of total SOC mineralized in fractions and bulk soil were declined under manure application further indicating high SOC stability under manure application.Taken together these findings concluded that,the long-term manure addition not only had the potential to sequester C but also could have less SOC losses by enhancing recalcitrant C forms as revealed by SOC mineralization.