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.