Changes In Soil Carbon And Nitrogen Accumulation And Mineralization Driven By Microbial Biomass Stoichiometry Under Long-term Fertilization In Typical Paddy Soils Of Southern China

Mineralization of soil organic carbon?SOC?and nitrogen?N?are the fundamental biogeochemical processes related to soil fertility and productivity.In addition,the SOC content controls the N availability in the soil.So sustaining SOC is of primary importance in terms of nutrients cycling and soil productivity.Moreover,accumulation of SOC is also important to reduce the global warming potential and achieving food security.Therefore,the presence of SOC ensures the N availability in the soil which is essential for soil fertility and crop productivity.However,the response of soil and microbial stoichiometry of carbon?C?,N and phosphorus?P?to mineralization and accumulation of SOC and N resulted from long-term fertilization regimes little known.Based on two field experiments with rice-cropping over three decades,the objectives of the study were to?i?evaluate the response of SOC and N accumulation and related soil microbial properties to long-term fertilizers,?ii?investigate the responses of SOC and N mineralization to changes in C,N,and P stoichiometry of soil and microbial resulted from long-term fertilization regimes.The soil was sampled from rice-based cropping systems subjected to long-term fertilization with different nutrient input amounts including?i?Wuchang site:control?CK?,N alone,N combined with mineral phosphorus?NP?,NP plus potassium?NPK?,manure alone?M?,and M combined with N?MN?,NP?MNP?,NPK?MNPK?,and a higher rate of M with NPK?hMNPK?,and?ii?Qiyang site:CK,NPK,M,PKM,NKM,NPM,and NPKM.For both sites,it was investigated nutrient availability,microbial biomass of C,N,P and its stoichiometry as well as associated extracellular enzymes related to C and N cycling.The sampled soil was incubated for the determination of SOC and N mineralization.Results showed that SOC and total N were higher under the combined manure and mineral fertilizer treatments relative to the CK and applications of mineral fertilizers treatments.Soil microbial biomass carbon?SMBC?,SMBN,and SMBP in both paddy soils were significantly higher in manure,combined manure and mineral fertilizers compare to the CK?P<0.05?.Manure application significantly decreased soil and microbial biomass C:P and N:P ratio than the treatment of NPK and CK?P<0.05?because of higher N and P availability.In the rice-wheat rotation system,C-acquiring enzymes activities decreased in manure amendments and increased under mineral fertilization treatments,and vice versa for N-acquiring enzymes.By contrast,in the double rice cropping system,the C-and N-acquiring enzymes activities significantly increased??-glucosidase by 66-201%,32-197%?,?cellobiohydrolase by166-419%and 310-701%?and?leucine aminopeptidase by 17-184%?,and?N-acetyl-glucosaminidase by 62-132%?compare to the NPK and CK,respectively.The average enzymatic stoichiometry of soil enzymes ln?BG?:ln?LAP+NAG?,ln?BG?:ln?AP?and ln?LAP+NAG?:ln?AP?was 0.36,0.16,0.47?Wuchang?and 0.75,0.19,0.27?Qiyang?,respectively.For Wuchang,relative to the CK and treatments with mineral fertilizers,treatments with manure?M,MN,MNP,MNPK,and hMNPK?significantly increased SOC and N mineralization by 48-78%and54-97%,respectively.Whereas,for Qiyang site,SOC and N mineralization increased by 36-79%and100-142%in NPK,M,and manure plus mineral fertilizers treatments than the CK,respectively.Significantly negative correlations were also observed between SOC and N mineralization and soil C:P and N:P ratio,as well as microbial biomass SMBC:SMBP and SMBN:SMBP stoichiometry,respectively.The leucine aminopeptidase?LAP?enzyme?Wuchang?,C-and N-acquiring enzymes?Qiyang?showed a significant positive correlation with SOC and N mineralization,and negatively related to the qCO₂.Lower soil elemental?C:P and N:P?and microbial biomass stoichiometry?SMBC:SMBP and SMBN:SMBP?and increase of LAP and/or C-and N-acquiring enzymes resulted from the combined application of manure and mineral fertilizers,accelerated SOC and N mineralization.On the other hand,the microbial metabolic quotient?qCO₂?decreased by 32-55%in treatments with manure compared to the N and NP treatments but showed no effect on the qCO₂ when compared to the NPK treatment.However,the availability of N and P had limited effects on the qCO₂ after reaching a certain value in rice-wheat(0.69-0.72 mg CO₂-C g^-1 MBC h^-1)and double rice system(0.88-1.14 mg CO₂-C g^-1MBC h^-1).Another experiment was conducted to evaluate the relationship between SOC and N mineralization,and OC fractions and their effects on crop productivity.Results showed that the combined application of manure and mineral fertilizers significantly increased the unprotected OC fraction?cPOC?by 44-72%compared to CK.Manure amendments increased physically protected??agg?,physico-chemically protected?H-?silt?,and physico-biochemically protected?NH-?silt?fractions of OC by 30-56%,62-150%,and 27-51%,respectively.Whereas chemically and biochemically protected fractions showed no response to manure application.Regression analysis revealed that accumulated unprotected?cPOC?,physically protected??agg?,and physico-chemically protected fraction?H-?silt?enhanced SOC and N mineralization.Accumulation of OC content in cPOC,?agg,and physico-chemically protected fractions?H-?silt,H-?clay?,and mineralization of SOC and N predominantly increased wheat,and rice grain yield under long-term manure additions.In conclusions,SOC and N mineralization had decreasing trends with an increase in soil and microbial biomass stoichiometry of C:P and N:P.This result was proven that microbial biomass stoichiometry depended on soil stoichiometry in a long-term field experiment.Therefore,soil stoichiometry could be considered as an indicator for SOC and N mineralization,implying that long-term manure application decreased soil elemental and microbial stoichiometry by enhancing the microbial biomass pool via abundant nutrients input.Therefore,mineral nutrients input with manure amendments could be an optimal strategy to meet the microbial stoichiometric demands and enhance nutrient availability for crops in agricultural ecosystems.In association with OC fractions,long-term manure and mineral fertilizers application enhanced OC accumulation in unprotected and physical protected fractions,which are labile pool for SOC and N mineralization for increasing carbon stabilization and productivity in paddy soils.