A greater source of methane from drainage rivers than from rice paddies with drainage practices in southeast China

Rice paddies and rivers draining agricultural watersheds have been documented as two major sources of atmospheric methane (CH4), while parallel flux measurements have been rarely taken on CH4 from rice paddies against the drainage rivers in agricultural watersheds. Moreover, the drainage events during the rice growing season deliver sufficient organic and inorganic nutrients to rivers, which makes there an increasing concerned source of atmospheric CH4. Here, we compared CH4 emissions from rice paddies and the associated rivers draining agricultural watersheds by a parallel field experiment in southeast China. Over the whole rice–growing season (June–November), CH4 fluxes from drainage rivers averaged 25 mg m–2 h–1, which were 82 % higher than those from rice paddies (13.7 mg m–2 h–1). Besides the dependence of riverine CH4 fluxes on water dissolved oxygen (DO), both ecosystem CH4 fluxes were significantly correlated with water/soil dissolved organic carbon (DOC), oxidation-reduction potential (Eh) and the microbial functional genes that drive CH4 production or uptake [methanogens (mcrA and methanogenic archaeal 16S rRNA) and methanotrophs (pmoA)]. Our results highlighted that the CH4 mitigation potential benefited from the drainage practice in rice paddies might have been totally offset by its induced increases in CH4 emissions from rivers draining agricultural watersheds at the regional scale. ●Rivers draining agricultural watersheds are an important source of atmospheric CH4.●CH4 fluxes from drainage rivers have been rarely related to the abundance of microbes.●CH4 fluxes from drainage rivers were 82 % greater than those from rice paddies.●The increase in CH4 emissions in rivers may offset the mitigation benefit of CH4 in rice paddies.