Freshwater Salinization Mitigated NO Emissions in Submerged Plant-Covered Systems: Insights from Attached Biofilms.

Submerged plants (SMPs) play a critical role in improving water quality and reducing NO greenhouse gas emissions. However, freshwater salinization represents a major environmental challenge in aquatic systems. To investigate the impact of salinization on NO emissions, this study conducted indoor mesocosm experiments simulating SMP and nonsubmerged plant (Non_SMP) areas in freshwater lakes. The objective was to explore the effects and microbial mechanisms of the attached biofilm on NO emission in freshwater salinization. Salinization systems (700-1500 μS cm) reduced NO flux by 37.0 and 40.5% compared to freshwater systems (<700 μS cm) of SMPs and Non_SMPs, respectively. Kinetic experiments showed that the reduction in NO emissions was mainly attributed to the attached biofilm rather than the sediment or water. The NO net emission rates of the attached biofilm decreased by 47.1 and 71.8% in salinization systems of SMPs and Non_SMPs, respectively, compared with freshwater systems. Additionally, biofilms in salinization systems exhibited lower denitrification rates. Furthermore, salinization reduced the NO production potential (( + )/( + )), thereby further decreasing NO emissions. This study provides valuable insights into the role and mechanisms of biofilms in mitigating NO emissions in salinized freshwater lakes.