Diffusive CH 4 Emissions From Agricultural Ditches Overshadow CH 4 Sinks by Upland Fields

Abstract Ditches are potentially important sources of methane (CH 4 ) in agricultural regions, but their CH 4 emissions are largely unknown due to data scarcity. Here, we investigated CH 4 concentrations and diffusive fluxes across different ditches in the North China Plain (NCP), an extensive upland agricultural region with maize‐wheat rotations, and well‐constructed ditch systems. Based on intensive monthly and extensive regional surveys, we found that (mean ± SD) CH 4 concentrations (11.42 ± 37.69 μmol L −1 ) and fluxes (344.7 ± 1,198.1 μmol m −2 h −1 ) in the agricultural ditches (ADs) showed high variability, primarily driven by spatial and temporal heterogeneity in nutrient and carbon inputs. On average, CH 4 concentrations and fluxes were 3–12 times higher than those in the nearby agricultural‐rural ditches (3.80 μmol L −1 , 99.8 μmol m −2 h −1 ) and rivers (0.92 μmol L −1 , 47.1 μmol m −2 h −1 ). Dissolved organic carbon (DOC) and ammonium (NH 4 + –N) were primary drivers of CH 4 emissions in the ADs, highlighting the key role of nutrient and carbon inputs from surrounding fields. The annual diffusive CH 4 emission from ADs in the NCP was estimated to be 1,836.3 ± 311.6 Gg CH 4 yr −1 and 68.1 ± 7.3 Gg CH 4 yr −1 based on the mean and median CH 4 fluxes, respectively, acting as a significant source of CH 4 emissions, despite large uncertainty. This emission overwhelmingly offsets the CH 4 uptake by soils (i.e., −9.2 Gg CH 4 yr −1 ) in the NCP, highlighting the necessity of including CH 4 emissions from ADs in estimating CH 4 budget from upland agricultural regions.