Mitigating gaseous nitrogen emissions in cotton fields through green manure and reduced nitrogen fertilization

Integrating green manure with reduced nitrogen (N) fertilization is a promising strategy to mitigate N emissions in intensive cotton cultivation, however, the underlying mechanisms remain poorly understood. This study investigated the effects of three green manure incorporation patterns-no green manure (NG), <i>Orychophragmus violaceus</i> (OVG), and <i>Vicia villosa</i> (VVG)-combined with four N reduction levels (100, 50, 25%, and conventional) on gaseous N emissions (NH₃ and N₂O), soil physicochemical properties, and bacterial community characteristics using a cotton field experiment in the Yellow River Basin. Results showed that OVG incorporation with 25% N reduction (N2 treatment) significantly reduced total gaseous N emissions by 36.07% on average during the cotton growth period, reducing NH₃ and N₂O emissions by 13.31-54.11% and 32.25-68.77%, respectively, compared with N2 application without OVG. OVG application also increased the relative abundance of Proteobacteria (28.10%), enhanced heterogeneous selection in bacterial community assembly (200%), and increased the complexity of co-occurrence networks, compared with NG. Compared with conventional N fertilization (N3 treatment), ≥50% N reduction significantly lowered NH₃ (>25.51%) and N₂O (>32.76%) emissions, reduced the relative abundance of Acidobacteria (-20.23%), simplified co-occurrence networks, and increased homogeneous selection in bacterial assembly (50.00%). Integrating green manure with 25% N reduction substantially reduced gaseous N emissions, which was associated with the enhanced microbial biomass carbon (MBC) and facilitated recruitment of key bacterial taxa (e.g., <i>Sphingosinicella</i>, <i>Azohydromonas</i>, <i>Phototrophicus</i>) within the microbial co-occurrence network. These findings provide insight into how green manure application coupled with N reduction can mitigate gaseous N losses and reshape soil microbial ecology, offering a theoretical basis for sustainable nutrient management during cotton production.