Freshwater Salinization Mitigated N₂O Emissions in Submerged Plant-Covered Systems: Insights from Attached Biofilms

Submerged plants (SMPs) play a critical role in improving water quality and reducing N₂O greenhouse gas emissions. However, freshwater salinization represents a major environmental challenge in aquatic systems. To investigate the impact of salinization on N₂O 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 N₂O emission in freshwater salinization. Salinization systems (700-1500 μS cm^-1) reduced N₂O flux by 37.0 and 40.5% compared to freshwater systems (<700 μS cm^-1) of SMPs and Non_SMPs, respectively. Kinetic experiments showed that the reduction in N₂O emissions was mainly attributed to the attached biofilm rather than the sediment or water. The N₂O 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 N₂O production potential ((nirS + nirK)/(nosZI + nosZII)), thereby further decreasing N₂O emissions. This study provides valuable insights into the role and mechanisms of biofilms in mitigating N₂O emissions in salinized freshwater lakes.