The role of symbiotic nitrogen-fixing bacteria, Rhizobium and Sinorhizobium, as "bridges" in the rhizosphere of legumes after fomesafen application

The widespread use of fomesafen, a diphenyl ether herbicide, has resulted in the accumulation of residues in soils of legume crop fields, potentially disrupting the dynamics of rhizospheric nitrogen-fixing microbial communities. However, such knowledge remains to be systematically pursued. This study employed field experiments to evaluate the impact of different fomesafen dosages on modulating nitrogen-fixing microbial community structure in rhizosphere of six legume species and their microbial network interactions across three seasons. The findings revealed that fomesafen significantly reduced the abundance of nifH genes in legume rhizospheres, with 16.7 %- 43.5 % of nitrogen-fixing genera showing significant changes in abundance. Species-specific recruitment of rhizospheric microbes by legumes with differing sensitivities to fomesafen, coupled with fomesafen dosage, played a pivotal role in shaping nitrogen-fixing microbial communities. Correlation analysis highlighted a strong relationship between changes in symbiotic nitrogen-fixing bacteria and plant health. Co-occurrence network analysis further demonstrated that fomesafen disrupted microbial interactions by reducing edge density and altering network connectivity. Nonetheless, symbiotic rhizobia, including Rhizobium and Sinorhizobium, accounted for 53.3 %-91.7 % of the connector module in the network, maintaining key roles in nitrogen cycling and herbicide degradation to support plant health and productivity. This study provides essential insights into the adaptation mechanisms of the rhizospheric nitrogen-fixing microbial community of different legume species to fomesafen stress, offering guidance for the development of sustainable practices that enhance legume nitrogen fixation and reduce soil herbicide residues.