Soil hydrolases and microbial biomass regulate wetland CO2 emissions under warming and water level reduction

Changes in temperature and water level affect the wetland CO₂ emissions, while the response mechanism of marsh wetland CO₂ emissions to temperature and water synergistic change is still rare. We monitored the CO₂ emissions and analyzed their relationships with plant and soil properties in a typical marsh wetland of Sanjiang Plain, Northeast China. We utilized the open-top chamber (OTC) passive warming combined with the automatic water level control platform. Four treatments including control (CK), warming (W), water level reduction (WR), combined warming and water level reduction (WRW) were established. The results showed that the CO₂ flux in the growing season reached the maximum value of 425.26 mg·m⁻²·h⁻¹ in the WRW treatment. Warming under different water conditions promoted CO₂ emissions. Water level reduction interacted with warming and intensified CO₂ emissions. Combined warming and water level reduction significantly enhanced soil hydrolases activities, bacteria and nirK gene abundances, microbial biomass carbon and nitrogen (MBC and MBN) contents. Mantel test results revealed that soil β-glucosidase (BG), acid phosphatase (AP) activities, MBC, MBN contents, plant TC and soil temperature had a significant positive effect on cumulative CO₂ flux. The regression analysis demonstrated air and soil temperatures, plant height, chlorophyll content of Carex lasiocarpa were critical factors influencing the dynamic of CO₂ emissions in the marsh wetland of the Sanjiang Plain. This finding underscores the essential influence mechanism of hydrolases and MBC, MBN contents on CO₂ emissions from marsh wetland under warming and water level reduction conditions. Mitigating climate warming and enhancing wetland water levels to inhibit hydrolytic enzyme activity may reduce wetland CO₂ emissions and increase carbon sink.