Turnover Of Crop Residue N And Relevant Microbial Ecology Characteristics In Mollisols

Crop residue amendment to soil is recommended as an effective management practice to balance soil organic carbon（SOC）stock,maintain the soil quality and level nutrient supply to crops.The crop rhizosphere is an interface between living roots and soil,considering as the hotspot of biochemical processes.Plant-C efflux alters the microbial activity and residue-nitrogen（N）mineralization.Thus,examining the role of microbes in the mineralization of N in the rhizosphere will improve the residue-N use in the subsequent crops and advance our knowledge about N cycle in farming soils;On the other hand,the effect of crop residues with different C/N ratios on soil organic carbon（SOC）stock and its stability depends on the functional response of soil microbial communities.Characterizing microbial functional gene profile and community composition during residue mineralization would improve our understanding on the microbe-driven decomposition of crop residue in farming soils.This study aimed to investigate the residue N mineralization processes and microecological characteristics in soils,which could improve residue management and N fertilizer formulation in cropping systems,and develop solutions to balancing SOC stock with crop residue return in agricultural systems.The results are as follows:（1）Using rhizobox system and ¹⁵N-labelled soybean and maize residues to investigate the microbiome associated with the residue-N mineralization in the rhizosphere.We found that residue-N was mineralized faster in the rhizosphere than the bulk soil,and from soybean residue than maize residue.The co-occurrence network of N-mineralization associated community altered with the major shift in the hubs of the network from Streptomyces in the no-residue control to Microlunatus and Novosphingobium in the soybean-residue treatment,and Burkholderia-Caballeronia-Paraburkholderia and Gaiellales in the maize-residue treatment.Moreover,greater enrichment of taxa against the unit of residue-N mineralization in the soybean than maize residue treatment was correspondent with the enriched ammonification genes, likely contributing to the enhanced mineralization of soybean residue-N in the rhizosphere.An increase in dissolved organic C and a decrease in available N concentration in the rhizosphere,relative to the bulk soil,might shift bacterial community favoring the residue-N mineralization.（2）Using microbial metagenomic analysis,Illumina Mi Seq and plant-¹⁵N labeling techniques,we characterized microbial functional gene profile and community composition,and subsequently assessed their contributions to residue-induced C accumulation in SOC pools.We found that residue amendment brought increases in the assemblage of genes involved in C-degradation and N mineralization.The N mineralization genes were correlated with the C and N accumulation in the particulate and mineral-associated C pools,and plant-derived aliphatic forms of SOC.（3）Based on a three-years pot experiment,we deployed ¹⁵N-technique combined with HT-q PCR to trace the residue N allocation in plant-soil-microbe continuum and assess the microbial metabolic contribution to the residue-N mineralization.We found that,residue amendment greatly improved the N₂ fixation compared to chemical fertilizer treatments,and the soybean residue had a stronger effect than maize residue.After three soybean growing seasons,the average recovery of the residue N by soybean was about 43.3%,which was significantly higher than maize residue amendment（37.0%）.The proportion of N mineralized from the soybean residue was significantly higher than that from the maize residue.Microbial metabolisms had important effects on the residue N mineralization.（4）As residue amendment stimulated the N₂ fixation in legume crops,the N₂fixing bacterial community in the rhizosphere of legume warrants further investigation.Using high-throughput sequencing of the nif H gene as a molecular marker for diazotrophs,we found that residue amendment increased the abundance of diazotrophs and fundamentally altered the composition of its community in the rhizosphere of soybean.It increased the relative abundances of Bradyrhizobium and Azohydromonas compared to the chemical fertilizer treatment.The copy number of nif H gene in the rhizosphere was associated with dissolved organic carbon and N₂ fixation.