Water regime governs how straw-derived carbon inputs influence microbial pathways responsible for N2O production in red paddy soils

Nitrous oxide (N 2 O) is a potent greenhouse gas, and paddy soils represent a major agricultural source. Straw return is widely adopted in rice systems, yet how different straw-derived carbon (C) forms regulate microbial N 2 O production pathways under contrasting water regimes remains poorly understood. Here, we investigated the effects of direct straw incorporation (ST), straw combined with microbial inoculant (IN), and straw-derived biochar (BC) on N 2 O production pathways in paddy soils under non-flooded and flooded conditions. 15 N– 18 O dual-isotope tracing technique, combined with quantitative PCR and metagenomic analyses, was used to quantify the relative contributions of nitrifier nitrification (NN), nitrifier denitrification (ND), nitrification-coupled denitrification (NCD), and heterotrophic denitrification (HD). Soil moisture was identified as the primary factor governing straw-derived C effects on N 2 O emissions. Under non-flooded conditions, heterotrophic denitrification dominated N 2 O production (65–83% of total flux across treatments), whereas treatment effects on N 2 O emissions were driven primarily by shifts in nitrification-related pathways. ST and IN significantly reduced N 2 O emissions by suppressing ND through nitrifying bacteria. In contrast, biochar elevated N 2 O emissions by enhancing NN via soil pH increase and AOB- amoA enrichment. Under flooded conditions, HD process became the overwhelming N 2 O source. ST and IN markedly increased N 2 O emissions by stimulating HD process through oxygen depletion and enrichment of denitrifying functional taxa (e.g., narIHG -Firmicutes). Biochar exerted a compensatory effect by concurrently enhancing nitrification-associated N 2 O production and nosZ -mediated N 2 O reduction, resulting in near-neutral net emissions. Collectively, straw-derived C forms induced distinct, moisture-dependent regulatory effects on N 2 O production pathways by reshaping N-cycle functional genes and key microbial communities. These findings provide a mechanistic framework linking moisture conditions, microbial processes, and N 2 O emissions in paddy fields.