Dynamic variability of soil diazotrophs in bulk-rhizosphere and phenological stages under long-term mulching in an eroded area in the Loess Plateau

Free-living diazotrophs play a significant role in the process of soil biological nitrogen fixation. Long-term field management has a cumulative effect on soil microbial communities. However, after long-term mulching measures in erosion areas, the stability difference in diazotrophs in bulk-rhizosphere soil and the temporal dynamic changes remain unclear. In this study, we analysed the dynamic variability of soil diazotrophic community structures using high-throughput sequencing. The result of Chao1 diversity index showed that mulching effectively increased the richness of soil diazotrophs. Combining the community composition, it was found that the diversity level of the soil diazotrophic community under mulching decreased more drastically along the development stages compared with no-tillage, and Bradyrhizobium played a major role in nitrogen fixation. In rhizosphere soil, mulching promoted the relative abundances of Azohydromonas, Bradyrhizobium, and Skermanella. Co-occurrence network analysis indicated that mulching measures strengthened the connection between the dominant components to improve the aggregation of the network. The partial least squares-path model showed that the richness of diazotrophs in rhizosphere soil had an indirect effect on mulching through the available carbon and nitrogen nutrients of bulk soil, thereby improving the response rate to environmental variables. Overall, our findings showed that under long-term mulching, the richness and aggregation of dominant components of the soil diazotrophic community increased with the phenological period and distance from roots, revealing the importance of mulching in eroded areas to improve farmland soil nitrogen fixation capacity. This study provides a theoretical reference for microecology to effectively improve soil fertility in similar areas.