Diffusive CH₄ Emissions From Agricultural Ditches Overshadow CH₄ Sinks by Upland Fields

Ditches are potentially important sources of methane (CH₄) in agricultural regions, but their CH₄ emissions are largely unknown due to data scarcity. Here, we investigated CH₄ 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) CH₄ 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, CH₄ 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 (NH₄⁺–N) were primary drivers of CH₄ emissions in the ADs, highlighting the key role of nutrient and carbon inputs from surrounding fields. The annual diffusive CH₄ emission from ADs in the NCP was estimated to be 1,836.3 ± 311.6 Gg CH₄ yr⁻¹ and 68.1 ± 7.3 Gg CH₄ yr⁻¹ based on the mean and median CH₄ fluxes, respectively, acting as a significant source of CH₄ emissions, despite large uncertainty. This emission overwhelmingly offsets the CH₄ uptake by soils (i.e., −9.2 Gg CH₄ yr⁻¹) in the NCP, highlighting the necessity of including CH₄ emissions from ADs in estimating CH₄ budget from upland agricultural regions.