Potential of submerged macrophytes restoration for reducing CH4 and CO2 emissions in a typical urban lake.

As one of the areas most affected by human activities, urban lakes play a crucial role in global carbon cycle. Currently, submerged macrophyte restoration is a common ecological practice in urban lakes, primarily aimed at improving water quality and enhancing the aesthetic value of lake environments. Despite its widespread application, its contribution to carbon emission reduction has not received sufficient attention. In this study, we quantified the fluxes, concentrations and isotope signatures of CH4 and CO2 in the restoration zone (RL), the unrestored zone (UR) and the inflow rivers (IR) of Lake Xuanwu, along with potential environmental and dissolved organic matter (DOM) factors over the course of a year. The results indicated that the restoration of submerged macrophytes significantly diminished the emission of CH4 and CO2 from the lake. Compared to the UR and IR zone, the CH4 flux in the RL zone was reduced by 82.27 % and 92.18 %, while the CO2 flux decreased by 464.95 % and 133.12 %, respectively. Further investigation revealed distinct eutrophication levels between the RL zone with submerged macrophytes compared to the UR zone, and higher eutrophication levels were associated with reduced carbon sequestration stability. Nitrogen and phosphorus played critical roles in the emission of CH4 and CO2, respectively. Submerged macrophytes directly reduce carbon emissions through photosynthesis and significantly influence the long-term carbon sequestration capacity of lakes by secreting oxygen, modifying the ecological characteristics of the aquatic environment, and altering the production and mineralization processes of CH4 and CO2 in sediment porewater. These results underscore the potential of submerged macrophytes restoration as a viable strategy for reducing local emissions of CH4 and CO2 in urban lakes.