Green manuring increases network complexity and deterministic assembly of diazotrophic community in a subtropical paddy soil

Chinese milk vetch (Astragalus sinicus L.) is widely used as green manure, fixing dinitrogen (N₂) during its growing season to enhance soil fertility and crop yields when incorporated into soils. Straw return is another common farmland management practice in paddy fields. However, how leguminous green manure and straw return influence free-living diazotrophic bacteria during the subsequent rice-growing season is not well understood. To address this, we conducted an eight-year rice-rice-green manure rotation and straw return experiment in southern China to assess the effects of green manuring and straw return on diazotrophic communities at various rice growing stages. The results showed that straw return increased diazotrophic abundance, whereas green manuring did not significantly affect it. Neither green manuring nor straw return influenced diazotrophic diversity, though both significantly altered diazotrophic community structure. Dissolved organic carbon, total nitrogen, nitrate and ammonium were identified as key factors shaping community structure. Green manuring decreased the relative abundance of Desulfomonilia by 31.1 % whereas straw return increased it by 18.7 %. Compared to the control, green manuring rather than straw return enhanced the complexity of diazotrophic co-occurrence network. While stochastic processes dominated diazotrophic community assembly in the rice paddy soils, green manuring increased the relative importance of deterministic process. Taken together, these findings suggest that green manuring may be a more effective management practice for shaping diazotrophic community dynamics in paddy soils. The influence of green manuring on deterministic processes and network complexity underscores its potential to enhance the stability and functionality of diazotrophic communities, potentially contributing to long-term soil health and agricultural sustainability.