Nighttime warming reshapes thermal regimes in freshwater wetlands: cascading effects on carbon dynamics

Abstract Microclimate can profoundly impact carbon (C) cycling in terrestrial ecosystems. However, due to the complex thermal conduction among air, water, and sediment, the responses of wetland microclimate to the driving forces of global change remain largely unexplored. Here, based on a two-year (2022-2023) field manipulative experiment in a freshwater wetland in the North China Plain, this study aimed to investigate the effects of simulated climate warming and atmospheric nitrogen (N) deposition on the temperatures of shallow-water, deep-water, and sediment. The nighttime warming effect increased the daily mean temperatures of the shallow-water, deep-water, and sediment layers by 0.71°C, 0.73°C, and 0.64°C, respectively. In addition, the diurnal temperature range of the deep-water layer was reduced by 0.17°C. In contrast, N addition did not affect the above parameters of shallow-water, deep-water, or sediment. The impact of N addition showed seasonal fluctuations. Warming-induced temperature increase was influenced mainly by solar radiation and water turbidity. Changes in submerged plant cover had a cooling effect at various vertical levels. In addition, increased wetland temperature may affect the rate of microbial metabolism to significantly increase the rate of carbon emissions. The present study offers empirical evidence concerning temperature changes at varying vertical levels within freshwater wetlands in the context of climate warming. Our findings emphasize the necessity of integrating data on shallow-water, deep-water, and sediment temperatures into the forecasting of wetland carbon cycling responses to global change.