Water regime alters microbial mechanisms of N₂O emission in metal-contaminated paddy soils

Microorganisms are essential for soil nitrous oxide (N₂O) emissions through participating in key nitrogen (N)-related processes. However, the effect of water regimes on the interactions between N₂O emissions and microbial processes in metal-contaminated soils is unclear. Here, we conducted a soil microcosm experiment with two water management strategies (non-flooding and flooding) and six metal addition treatments including low (2 and 200 mg kg^-1) and high (10 and 1000 mg kg^-1) levels of individual and combined Cd and Cu. The effects of high levels of individual Cd and Cu contamination on soil N₂O emissions varied depending on water regimes, showing antagonistic effects under non-flooding conditions and synergistic effects under flooding conditions. High levels of co-contamination significantly inhibited nitrification under both water regimes, primarily due to reduced abundance of Nitrosospira. In contrast, this co-contamination decreased the abundance of Ramlibacter, thereby inhibiting denitrification and dissimilatory nitrate reduction to ammonium (DNRA) under flooding conditions. The inhibition of these key microorganisms and their mediated N-cycle processes reduced soil N₂O emissions under both water regimes. This reduction was greater under flooding conditions because more N-related processes were inhibited. Metagenomic binning further indicated that Nitrosospira carried nitrifying genes, while Ramlibacter contained genes involved in denitrification, assimilatory nitrate reduction to ammonium (ANRA), and DNRA. These findings implied that both microorganisms had potential to produce N₂O. Overall, water management strategies and metal contamination altered the microbial processes of N₂O emissions, highlighting the importance of appropriate water management in mitigating greenhouse gas emissions from metal-contaminated paddy soils in southern China.