Microbial role in enhancing transfer of straw-derived nitrogen to wheat under nitrogen fertilization

Straw return plays an important role in improving soil fertility and crop yield. Straw combined with nitrogen (N) fertilizer can alleviate soil nutrient stress caused by N competition between soil microorganisms and crops during straw decomposition. Nevertheless, the microbial mechanisms by which N application regulates the allocation of straw-derived carbon (C) and N in soil-plant systems remain unresolved. In this study, we explored microbial roles in straw nutrient release and transfer under N fertilization using a pot wheat experiment with four treatments (i.e., no straw and no N fertilizer, S0N0; N fertilizer alone, S0N1; 13C/15N double-labeled maize straw alone, S1N0; and 13C/15N double-labeled maize straw plus N fertilizer, S1N1). We found that S1N1 significantly increased wheat above-ground biomass, by 16.69-43.82%, compared with the other treatments. Soil mineral N contents (NO3--N and NH4+-N) in S1N1 gradually decreased over time, whereas more abundant soil microbial biomass N (MBN) and dissolved organic N (DON) were detected at the later stage of wheat growth. This result, which suggested a persistent N supply potential under straw return accompanied by N fertilization, was supported by evidence that the transfer of straw N to plants in S1N1 was 1.92 times higher than that in S1N0. Moreover, S1N1 strongly stimulated the proliferation of soil Actinobacteria, Proteobacteria, and Ascomycota and increased microbial network complexity. Five bacterial biomarkers (Micrococcaceae, Kocuria, Pontibacter, Actinotalea, and Limnobacter) and two fungal biomarkers (Schizothecium and Lasiosphaeriaceae) were detected in S1N1 at the early stage, whereas biomarkers newly abundant in the late stage of S1N1 were more diverse. According to functional prediction analysis, these S1N1-induced microbiota were associated with cellulolysis, chitinolysis, and N-cycling. Partial least squares path modeling revealed that N fertilization-induced regulation of the distribution of straw-derived C and N was mainly through the bacterial community, not the fungal community. Overall, these results suggested that chemical N fertilization promoted the release of straw-derived N and their transfer to crops by regulating microbial community composition, metabolic functions, and microbial biomass turnover.