Flash drought promotes decoupling of wetland soil CO2 emissions and CH4 uptake

Emerging drought regimes with higher frequency and rapid-onset reshape soil carbon interactions between carbon dioxide (CO2) emissions and methane (CH4) uptake, yet current research inadequately addresses how such shifts alter CO2-CH4 flux synergies. We conducted in situ measurements across drought intensity gradients during two extreme drought events in Dongting Lake wetland, a drought-vulnerable yet understudied wetland ecosystem, capturing real-world CO2-CH4 flux covariations. Complementary mesocosm experiments simulated two policy-relevant drought scenarios (slow drought vs. flash drought), isolating hydrological drivers of carbon flux coupling/decoupling from environmental factors to microbial community restructuring. Key findings: 1) Nonlinear thresholds govern CO2-CH4 coupling in wetland soils, with extreme drought inducing decoupling at seasonally distinct moisture critical points; 2) Mesocosm trials reveal a mechanistic divergence: CO2-CH4 flux coupling maintain under gradual moisture decline, whereas decoupling occurs under flash drought at the soil moisture threshold of 0.23 m3/m3; 3) Drivers at each drought stage were identified—while edaphic factors predominated during the pre-decoupling phase, the decoupling process was primarily mediated by microbial community shifts, notably a marked enrichment of Methylocystis and a concomitant reduction in Proteobacteria abundance. This study elucidates how drought dynamics reshape soil greenhouse gas coupling, providing insights to strengthen climate models and adaptive wetland management.