Microbial mechanisms underlying the reduction of N2O emissions from submerged plant covered system

Submerged plant (SP) restoration is a crucial strategy for restoring aquatic ecosystem. However, the effect of SP on nitrous oxide (N₂O) emissions remains controversial, and the impact of SP-attached biofilms on N₂O emissions is often overlooked. In this study, SP and non-submerged plant (NSP) systems were set up and operated continuously for 189 days, revealing that SP reduced N₂O flux by 42.4 %. By comparing the N₂O net emission rates from water, sediment, and biofilms, we identified biofilms as the primary medium responsible for the reduction in N₂O emissions in both SP and NSP systems. Further analysis of N₂O metabolic rates from nitrification, denitrification, and abiotic processes under light and dark conditions confirmed that counter-diffusion of dissolved oxygen and nutrients in SP biofilms plays a key role in reducing denitrification-driven N₂O emissions. Additionally, SP-attached biofilms increased <i>nosZII</i>-type denitrifiers (e.g., <i>Bacillus</i>) and reduced N₂O production potential ((<i>nirS</i>+<i>nirK</i>)/(<i>nosZI</i>+<i>nosZII</i>)). Notably, the establishment of a SP restoration project in a typical eutrophic freshwater lake demonstrated that SP could reduce N₂O fluxes by 61.5 %. This study provides significant insights for strategies aimed at mitigating N₂O emissions.