Root organic exudate stimulation of greenhouse gases is offset by radial oxygen loss in mineral-dominated anoxic wetland soil

• Root carbon release stimulates soil microbes and greenhouse gases. • Root oxygen release reduces CH 4 and CO 2 emissions. • Combined traits decrease CH 4 emissions and stabilize carbon. Wetlands are important contributors to global CH 4 emissions, yet uncertainties remain regarding plant-mediated controls of greenhouse gas (GHG) fluxes. Wetland graminoids influence these emissions via multiple pathways. The study focuses on two key root traits: organic carbon exudation and radial oxygen loss (ROL). Their individual and combined impacts on soil organic carbon (SOC) dynamics and GHG emissions were investigated in 26-day-long incubations, using an anoxic mineral-rich permafrost fen soil as a wetland proxy. Organic exudation was imposed by repeated injection of an artificial organic compound mix mimicking real root exudate, and ROL by continuous ambient-air injection. Organic exudation enhanced CO 2 and CH 4 emissions 3- and 2.2-fold, respectively. This stimulated fermentation, methanogenesis, and Fe mineral dissolution, thereby co-mobilizing SOC from the ‘rusty carbon sink’. ROL promoted methanotrophs and aerobes reducing bioavailable carbon pools, so that 29% less CO 2 and 47% less CH 4 were emitted compared to the no-addition control. The combination of organic exudation and ROL increased CO 2 by 70% and decreased CH 4 by 20% relative to the no-addition control, indicating active methanotrophs. Thus, Fe-associated OC increased ∼4.6-fold with an OC:Fe ratio above 1, suggesting a persistent ‘rusty carbon sink’ with less bioavailable SOC. This study demonstrates that combined root traits can exert opposing effects on CO 2 and CH 4 emissions, which can improve prediction and management for natural, agricultural, and restored wetlands.