Transformation Of Carbon And Nitrogen Forms In Decomposed Crop Straw Under Anaerobic And Aerobic Condition As Well As Its Effect On SOC Composition And Microbial Community

Crop straw incorporation is an important measure for recycling nutrient resources,ameliorating soil fertility and improving crop yield,which is of significance to ensure food security and agricultural sustainable development in China.However,the process of carbon and nitrogen compounds transforming in crop straw-soil organic carbon-soil microbe system as well as its potential mechanisms are still unclear.The laboratory and field experiments were conducted to investigate the transformation and potential mechanisms of carbon(C)and N(nitrogen)forms during the crop straw decomposition in crop straw-soil organic carbon-soil microorganism system using solid-state nuclear magnetic resonance(NMR)and high-throughput sequencing techniques.This study focused on the following aims:(1)the fixation of fertilizer nitrogen during wheat straw decomposition,(2)the transformation of carbon compounds during wheat straw decomposition,(3)the effect of wheat straw decomposition on the fractions and chemical structure of soil organic carbon,(4)the effect of crop straw incorporation on soil bacterial community composition and diversity,(5)the effect of crop straw incorporation on soil fungal community composition and diversity.The main findings are shown as follows:1.Nitrogen(N)from applied chemical fertilizers can be immobilized by crop straw decomposition.The immobilization of fertilizer N under aerobic conditions was greater than that under anaerobic conditions(2.7 mg g-1 vs 2.0 mg g-1).Under anaerobic conditions,most of the immobilized fertilizer 15N was protonated amide N.However,the immobilized fertilizer 15N tended to occur in more diverse N functional groups under aerobic conditions,including peptide-amide N(53%),N-methyl-containing amides(22%),amine N(5%),anilides N(7%)and heterocyclic N(2%).Under aerobic condition,a substantial part(54%-79%)of the immobilized fertilizer N is in labile forms,which could extend the effective residence time of fertilizer N and increase crop N use efficiency when they are remineralized in soil.2.The residual mass of wheat straw sharply decreased(37%-68%)in the first 90 days,and then slowly decreased(4%-16%)in the following 270 days under anaerobic and aerobic conditions.The decomposition rate constant(k)of wheat straw was lower under anaerobic conditions(0.022 d-1)than that under aerobic conditions(0.028 d-1),reflecting the slower decomposition of wheat straw in anaerobic than in aerobic conditions.The proportion of O-alkyl C in wheat straw decreased from 46%to 29%and 40%,respectively,under aerobic and anaerobic conditions after 360-day incubation.The proportion of aromatic Cs in wheat straw increased from 17%to 31%and 19%,respectively,under aerobic and anaerobic conditions.The relative proportions of nonprotonated aromatic C in wheat straw increased from 32%to 36%after 360-day decomposition,but there was not significantly changed after anaerobic decomposition.The 2D DQ/SQ NMR results showed that the increase of nonprotonated aromatic C in degraded wheat straw residues was mainly due to the formation of fused ring aromatic C structure.The long-range 13C-1H dipolar dephasing NMR results showed that about one gram of fused-ring aromatic C was produced per kilogram of wheat straw residue after one year of aerobic degradation.It is estimated that about 5.4 Tg C fused ring aromatic C could be produced worldwide from crop straw and root decomposition annually.3.Wheat straw decomposition increased the SOC,POC and MAOC under both aerobic and anaerobic conditions,and the MAOC under aerobic condition was significantly larger than that under anaerobic condition(P<0.05).Under the aerobic condition,wheat straw decomposition increased the proportion of O-alkyl C in SOC,POC and MAOC fractions(P<0.05).In contrast,wheat straw decomposition increased the proportion of alkyl C in SOC,POC and MAOC fractions under anaerobic condition(P<0.05).A/O-A and HB indices of SOC,POC and MAOC were higher under anaerobic than those under aerobic conditions(P<0.05).These results indicate that SOC,POC and MAOC would be considered more biochemically stable under anaerobic conditions than that under aerobic condition.4.SOC,DOC,TN,alkali-hydrolyzable N,available P and available K were significantly increased by 32.6%-55.8%,72.9%-217.3%,42.8%-57.1%,68.5%-81.8%,265.5%-481.3%and 31.0%-63.3%under F,WF,MF and WMF treatments(P<0.05),respectively,but soil pH were significantly decreased by 12.9-17.8%(P<0.05).Compared with F treatment,SOC and DOC were further increased by 14.3-17.4%and 44.4%-83.6%,respectively,under WF,MF and WMF treatments(P<0.05).However,there is no significant difference in SOC,TN,alkali-hydrolyzable N and available K between WF,MF and WMF treatments(P>0.05).Compared with CK treatment,F significantly increased the relative abundance of Proteobacteria and Actinobacteria,but significantly decreased the relative abundance of Acidobacteria,and decreased the bacterial richness,evenness and diversity(P<0.05).Compared with F treatment,WF increased the relative abundance of ?-Proteobacteria,Sphingobacteriia and Acidobacteria_Gp6,and increased the bacterial evenness and diversity(P<0.05).However,MF has little effects on bacterial diversity and evenness relative to F treatment.RDA and SEM results showed that the change of bacterial community composition and diversity were significantly correlated with the chemical structure of soil labile organic carbon fractions(LFOC,c-POC and f-POC).Compared with F treatment,wheat straw incorporation could increase the carbohydrate C in soil labile organic carbon,which has a significant correlation with soil bacterial community composition and diversity.5.F significantly increased the relative abundance of Ascomycota,but significantly decreased the relative abundance of Bosidiomycota,and decreased the fungal richness,evenness and diversity(P<0.05).Compared with F treatment,MF increased the relative abundance of Agaricomycetes and and increased the fungal evenness and diversity(P<0.05).However,WF has little effects on fungal diversity and evenness relative to F treatment.RDA and SEM results showed that the change of fungal community composition and diversity were significantly correlated with the content and chemical structure of c-POC and f-POC fractions.Compared with F treatment,Maize straw incorporation could increase the c-POC and f-POC fractions and their lignin C component,which has a significant correlation with soil fungal community composition and diversity.