Soil Moisture Reduction Accelerates CO ₂ Emissions in Near‐Saturated Cold and Wet Ecosystems

Abstract Global cold and wet ecosystems, such as permafrost, northern peatlands, Arctic tundra, boreal wetlands, and alpine swamp meadows, store large amounts of soil organic carbon (SOC) and are typically water‐rich. While it is well recognized that these ecosystems are highly vulnerable to climate warming as it accelerates SOC decomposition, how soil water levels regulate SOC decomposition and CO 2 emissions specifically by constraining oxygen (O 2 ) availability during the growing season remains poorly understood at large spatial scales. Here, we integrate field observations, global data sets, and process‐based models to quantify how soil water dynamics influence CO 2 emissions by regulating O 2 diffusion across these ecosystems. Results from 107 field sites consistently reveal a protective effect of high soil water levels against SOC decomposition and CO 2 release during the peak growing season (representing one‐third of the year), suggesting that, beyond warming, the loss of this protection due to soil moisture reduction under near‐saturated conditions is a critical driver of increased CO 2 emissions. However, global data‐driven data sets and process‐based model simulations show divergent correlations between soil CO 2 emissions and soil water levels. Many models, which rely on simplified soil moisture scalars or empirical functions to represent soil water level effects, inadequately reproduce the effects of soil water levels on SOC decomposition and CO 2 emissions in cold and wet ecosystems. Our study underscores the urgent need to incorporate more mechanistic representations of soil water‐mediated SOC decomposition into models and to improve spatially explicit hydraulic parameters, thereby enhancing projections of soil carbon‐climate feedback.