Moisture-dependent silicon reduces N2O emission: Insights from microbial community structure and soil properties in paddy soil

Silicon (Si) fertilizer application has been observed reduce nitrous oxide (N₂O) emissions in paddy soil. However, the responses of microbial communities remain poorly understood. In this study, we conducted a microcosm experiment with three Si application levels: CK (0 mg kg⁻¹, SiO₂), Si1 (100 mg kg⁻¹, SiO₂), and Si2 (200 mg kg⁻¹, SiO₂) under two soil moisture conditions-Flooding (125 % w/w) and Drying (60 % w/w). The abundance and the microbial community structure of nirK, nirS for N₂O production, nosZ I and nosZ II for N₂O reduction were analyzed. Under flooding condition, Si2 reduced N₂O emissions by 87.1 %, this mitigation effect weakened to 52.9 % under drying. Microbial community structure rather than size was regulated by Si application. N₂O-reducing microbial communities were strongly influenced by Si amendment, only nirS-containing denitrifier was observed in N₂O production process. Relative abundance of genus Noviherbaspirillum was typically depressed by Si application and positively correlated with N₂O flux. Several genera, such as Pleomorphomonas, Achromobacter, Intrasporangium were identified as crucial mediators of N₂O reduction process. Co-occurrence network analysis indicated greater complexity in the N₂O-reducing microbial communities in response to Si application. PLS-SEM reveals different mechanisms of Si depress N₂O flux in different soil moisture. Under flooding condition, the N₂O mitigation effect of Si was primarily driven by microbial processes. While under drying condition, the mitigation effect was dominated by a strong abiotic-chemical pathway. These insights contribute to a mechanistic understanding of how Si management can be optimized for N₂O mitigation in paddy soil.