Flooding lowers the emissions of CO2 and CH4 during the freeze-thaw process in a lacustrine wetland

The frequency and strength of soil freeze–thaw alternation have been increasing with the aggravation of climate change, which will affect soil physicochemical properties and biological characteristics and in turn alter soil carbon (C) emissions. The emissions of carbon dioxide (CO2) and methane (CH4) are often influenced by the water level in wetland ecosystems. However, the effects of soil freeze–thaw alternation under different water levels on soil C emissions in wetland ecosystems are still not clear. We examined the impacts of soil freeze–thaw alternation under different water levels on the emission of CO2 and CH4 in a wetland dominated by Phragmites australis, and the soil microbial community structure and enzymatic activities were studied. The results showed that freeze–thaw treatments promoted soil carbon emissions, and flooding significantly lowered soil CO2 and CH4 emissions under freeze–thaw treatments. The soil water content (SWC), dissolved organic carbon (DOC), and the activities of β-N-acetyl-glucosaminidase (NAG), polyphenol oxidase (PPO), and peroxidase (PER) were the main predictors of soil carbon emissions under freeze–thaw treatments. Flooding played a direct role in carbon emissions during the freeze–thaw processes, and the interpretation rates of soil CO2 and CH4 emissions were 65% and 37%, respectively. The high-water level indicated fewer carbon emissions during the freeze–thaw period. Therefore, appropriately raising the water level is conducive to reducing carbon emissions from wetlands in areas of seasonal frost.