Warming effects on the emission pathways of N2O and CH4 and the role of water column in shallow eutrophic lakes.

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Shallow lakes are frequently reported to be pivotal sources of greenhouse gases (GHGs) such as methane (CH4) and nitrous oxide (N2O) to the atmosphere, and their emissions are strongly influenced by changing climate and water eutrophic state. However, the transfer of N2O and CH4 from sediments to the atmosphere and the role of the water column in shallow eutrophic lakes remain poorly understood, particularly under warming conditions. Herein, the effects of experimental warming on diffusion and ebullition emissions of N2O and CH4 from shallow lakes and the potential drivers were investigated. Results showed that 88.68% of N2O emissions depended on diffusion, while 61.60% of the CH4 was emitted by ebullition. Warming significantly stimulated N2O and CH4 emissions at the water-air interface, with CH4 (0.45eV) having a higher temperature dependence than N2O (0.25eV). Warming also shifted CH4 emission pathways from diffusion-dominated to ebullition-dominated at approximately 20°C. As a source, the water column contributed 35.33%-66.51% of N2O emissions to the atmosphere, but as a sink, it oxidized 30.00%-67.49% of the CH4 from the sediments. These were driven mainly by the eutrophic state, except for the direct effect of warming, such as the changes of dissolved oxygen, organic carbon, and ammonia nitrogen in the sediment and water column. Warming not only accelerated the GHGs emission from sediments, but also correspondingly changed the transferring processes of GHGs in the water column and then the emissions to the atmosphere. Understanding the complex interactions between climate warming and N2O and CH4 fluxes in shallow eutrophic lakes is critical for effective lake management and control of GHGs emissions.