Microbial denitrification responses to elevated CO2 in lake-shore sediments under different flooding conditions.

The lake-shore zone is a critical site for recognizing the ecological impacts of global climate change, such as persistent elevated CO2 (eCO2). However, the direct effect of eCO2 on functional microorganisms in lake-shore sediments is neglected or confused at previous studies. In this study, the short-term direct effects of eCO2 on microbial denitrification in lake-shore sediments were investigated by simulating four flooding conditions (non-flooding (NF), intermittent flooding (IF), alternating high and low water-level flooding (HLF), and constant water-level flooding (WF)) through incubation experiments in the laboratory. This work demonstrated eCO2 directly increased denitrification in lake-shore sediments under different water-level conditions by different regulatory mechanism within the short term of 24 days. The microbial pentose phosphate pathway (PPP) and glycolysis in HLF and WF sediments were enhanced, while glycolysis in NF and IF sediments were obviously suppressed. Additionally, eCO2 strengthened negative associations of nosZ-type and nirK-type denitrifiers communities in NF, while increased their positive associations in IF, HLF, and WF. The regulation of metabolic pathways in denitrifiers would become more important than their microbial community when denitrifiers adapting eCO2. And the rising CO2 levels shifted sediments hotspots for potential N2O emissions from dry-wet alternation areas in lake-shore zone to the near-lake regions and lake basins. Based on these results, we recommend adjusting water-level management strategies in the lake-shore zone to address the greenhouse effect. For example, increasing the size of intermittently flooded areas and decreasing the proportion of continuously flooded areas in the lake-shore zone to mitigate N2O emissions and optimize sediment denitrification.