Soil CO2 not CH4 flux determines soil carbon emission response to seasonal precipitation variation in a brackish wetland

Global climate change-induced alterations in precipitation patterns have introduced uncertainty regarding soil carbon sequestration capacity in brackish wetlands. To investigate the effect of seasonal precipitation distribution (SPD) on soil carbon emissions, we conducted a field experiment in a brackish wetland in the Yellow River Delta, maintaining consistent annual precipitation but varying SPD (+73%, +56%, CK, –56%, and –73%). Increased precipitation during the spring was followed by decreased precipitation in the summer and fall (+73% and +56%), whereas decreased spring precipitation was followed by increased summer and fall precipitation (–56% and –73%). Precipitation remained consistent across all treatments during winter. The results revealed significant seasonal and inter-annual sensitivity of soil CO₂ fluxes to SPD, with the spring precipitation enhancement (+56%) treatment exerting a greater influence on emissions than the +73% treatment. In contrast, soil CH4 fluxes exhibited no statistically significant variations across seasons or in response to precipitation adjustments. Furthermore, hydrological mediation of SPD established inverse water-salt dynamics: increased precipitation in spring mitigated soil salinity, promoting vegetation colonization and growth, while reduced precipitation in summer and autumn alleviated inundation pressure, enhancing vegetation productivity. Increases in soil CO₂ fluxes driven by SPD were primarily attributed to alleviation of salinity stress and vegetation-mediated carbon partitioning, whereas CH₄ fluxes remained statistically constant across precipitation regimes. Therefore, we conclude that SPD predominantly affects soil carbon emissions in the brackish wetland by modifying soil CO2 fluxes. These findings provide mechanistic insights for refining predictive models of wetland carbon cycling under climate-driven precipitation reconstruction.