Objectives Nitrogen-fixing bacteria are an important functional group in the soil microbial community. The changes in the diversity and structure of nitrogen-fixing bacterial community can affect soil nitrogen fixation and nitrogen cycle dynamics. Here, we explored the mechanism of the diversity and structure of rhizosphere nitrogen-fixing bacterial community in regulating peanut yield under the different straw returning treatments.

Methods We conducted the long-term field experiment with different types of straw returning in peanut mono-cropping system at the Yingtan National Agroecosystem Field Experiment Station of the Chinese Academy of Sciences in Jiangxi. The field experiment included five treatments: no fertilization (CK), conventional NPK (NPK), NPK with straw (NPKS), NPK with straw and pig manure (NPKSM), and NPK with straw biochar (NPKB). The illumina sequencing of nifH gene was used to investigate the diversity and structure of nitrogen-fixing bacterial community in the rhizosphere.

Results The treatments with straw returning significantly promoted soil fertility including soil organic carbon (SOC), available K, total P, available P, total N, with the highest values under the NPKSM treatment. Compared with the CK and NPK treatments, the NPKS and NPKSM treatments significantly increased the diversity of soil nitrogen-fixing bacteria and shaped community composition. Alphaproteobacteria (82.5%) was the dominant class in the nitrogen-fixing bacteria community, and Bradyrhizobium (51.9%) was the dominant genus. Soil available P was the most important predictor of the nitrogen-fixing bacterial diversity, while pH, SOC, available K, total P, available P, total N and ammonia nitrogen (NH₄⁺-N) were the significant predictors of the nitrogen-fixing bacterial community composition. Structural equation modeling suggested that available P and total N was indirectly associated with peanut yield by changing the structure of nitrogen-fixing bacterial community.

Conclusions Straw returning significantly improved soil fertility, and available P was an important factor in driving the nitrogen-fixing bacterial community composition and peanut yield by enhancing the relative abundance of Deltaproteobacteria. Taken together, our study provides the scientific basis for establishing reasonable measures of straw returning to promote the potential of biological nitrogen fixation, and to improve the fertility and health of red soil.