Microbial mechanisms underlying the reduction of NO 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 (NO) emissions remains controversial, and the impact of SP-attached biofilms on NO 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 NO flux by 42.4 %. By comparing the NO net emission rates from water, sediment, and biofilms, we identified biofilms as the primary medium responsible for the reduction in NO emissions in both SP and NSP systems. Further analysis of NO 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 NO emissions. Additionally, SP-attached biofilms increased -type denitrifiers (e.g., ) and reduced NO production potential ((+)/(+)). Notably, the establishment of a SP restoration project in a typical eutrophic freshwater lake demonstrated that SP could reduce NO fluxes by 61.5 %. This study provides significant insights for strategies aimed at mitigating NO emissions.