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

Abstract

Changes in micro-climate are important drivers influencing carbon cycles in natural ecosystems. However, due to the complexity of thermal conduction among air, water, and sediment and the difficulty of accurately characterizing microclimatic impacts, the responses of wetland carbon cycles to microclimate under global change remain largely unexplored. In this study, based on a two-year field manipulative experiment from 2022 to 2023 in a freshwater wetland in the North China Plain, we investigated the effects of simulated nighttime warming and atmospheric nitrogen deposition on the temperatures of shallow water, deep water, and sediment, and also explored the potential influence of microclimate on wetland carbon cycles. The results showed that nighttime warming increased the daily mean temperatures of the shallow-water, deep-water, and sediment layers by 0.71, 0.73 and 0.64 °C, respectively. additionally, the diurnal temperature range of the deep-water layer decreased by 0.17 °C. Nitrogen addition have no significant effect on the aforementioned temperature parameters. However, its influence could be observed in the seasonal fluctuations of these layers to some extent. Moreover, the warming effect on these thermal regimes were primarily influenced by solar radiation and water turbidity. Submerged plant cover exerted a cooling effect at various vertical levels. Therefore, future increases in wetland temperatures may enhance the rate of microbial metabolism, likely resulting in elevated carbon emissions. This study provides empirical evidence of temperature changes across vertical layers in freshwater wetlands under climate warming. Our findings highlighted the need to incorporate shallow-water, deep-water, and sediment temperature data into wetland carbon cycling to better predict the responses of wetland carbon cycle to global change.