Straw return of maize and soybean enhances soil biological nitrogen fixation by altering the N-cycling microbial community

Crop straw return can enhance soil biological nitrogen (N) fixation (BNF) by improving soil properties, but the impacts of several crop straw mixture return on BNF and the N-cycling microbial community and their relationships are still unclear. Here, we investigated BNF and N-cycling microbial communities in soil with maize straw and maize-soybean straw mixtures, respectively. The results showed that soil BNF was highest (2.51 μg g−1 DW) in the 10 mg g−1 straw mixture treatment, being 25.1-fold and 250-fold higher than that in the 10 mg g−1 maize straw treatment and control, respectively. The copy numbers of nifH, nirS and nosZ significantly increased, and the relative abundances of the dominant microbial communities carrying the above genes were remarkably improved by 10 mg g−1 straw mixture return. The increase in soil BNF after straw return was significantly associated not only with N-fixing microbial communities but also with other N transformation microbial communities. In addition, 7, 8, 11 and 5 genera for the chiA, nirK, nirS and nosZ microbial communities, respectively, were positively associated with BNF. Therefore, straw return affected the expression of the functional genes and microbial communities involved in soil N-cycling by altering the soil properties (soil organic matter, pH, NO3− and available potassium), which ultimately enhanced the soil N fixation rate. Moreover, soil properties and N-cycling functional microbial communities have the largest common explanation (58.6%) for the differences in BNF. Our findings contribute to a better understanding of the mechanistic link between N-cycling microbial communities and BNF improvement after straw return, which provides theoretical guidance for the sustainable development of agriculture.