Seasonal Drought Reduces Carbon Sequestration in Coastal Wetlands

ABSTRACT Coastal wetlands represent one of the most intensive carbon sinks, and the stability of their soil carbon reserves is critical to the global carbon balance. While existing studies have predominantly focused on the effects of changes in total annual rainfall, the intensification of climate change has led to increasingly pronounced intra‐annual rainfall variability (e.g., greater concentration of rainy seasons and more frequent dry spells). However, systematic experimental evidence on how such variability regulates coastal wetland soil carbon storage remains lacking. We conducted a 5‐year field experiment in a Chinese coastal wetland, manipulating the timing of growing‐season rainfall while keeping the total amount constant. This created strong early‐growing season gradients in soil moisture and salinity, whereas mid‐ to late‐season heavy rainfall and groundwater recharge largely equalized surface soil moisture across treatments. Early‐growing season drought reduced topsoil organic carbon by about 9%–13%, whereas adding rain early in the season had no significant effect. The decline in soil organic carbon (SOC) was linked to reduced plant diversity, root biomass, and carbon inputs to both short‐lived and more stable soil pools. Scaled across China's coastal wetlands, this SOC decline could amount to roughly 2.2–3.2 million tons of CO 2 ‐equivalent, highlighting its potential impact. Our experiment therefore offers a globally relevant test case for understanding how climate variability constrains carbon sequestration. These results show that even short‐term drought can erode the climate‐mitigation potential of coastal wetlands by suppressing root‐driven carbon inputs, emphasizing the need to manage freshwater inflows and vegetation composition to maintain soil carbon storage under a changing climate.