Spatiotemporal variations of dissolved CH4 concentrations and fluxes from typical freshwater types in an agricultural irrigation watershed in Eastern China

Inland freshwater ecosystems are of increasing concerns in global methane (CH₄) budget in the atmosphere. Agricultural irrigation watersheds are a potential CH₄ emission hotspot owing to the anthropogenic carbon and nutrients loading. However, large-scale spatial variations of CH₄ concentrations and fluxes in agricultural catchments remain poorly understood, constraining an accurate regional estimate of CH₄ budgets. Here, we examined the spatiotemporal variations of dissolved CH₄ concentrations and fluxes from typical freshwater types (ditch, reservoir and river) within an agricultural irrigation watershed from Hongze catchment, which is subjected to intensive agricultural and rural activities in Eastern China. The dissolved CH₄ concentrations and fluxes showed similar temporal variations among the three freshwater types, with the highest rates in summer and the lowest rates in winter. The total CH₄ emission from this agricultural irrigation watershed was estimated to be 0.002 Gg CH₄ yr⁻¹, with annual mean CH₄ concentration and flux of 0.12 μmol L⁻¹ and 0.58 mg m⁻² d⁻¹, respectively. Diffusive CH₄ fluxes varied in samples taken from different freshwater types, the annual mean CH₄ fluxes for ditch, reservoir and river were 0.31 ± 0.06, 0.71 ± 0.13 and 0.72 ± 0.25 mg m⁻² d⁻¹, respectively. Among three freshwater types, the CH₄ fluxes were the lowest in ditch, which was associated with the lowest responses of CH₄ fluxes to water dissolved oxygen (DO), nitrate nitrogen (NO₃⁻-N) and sediment dissolved organic carbon (DOC) concentrations in ditch. In addition, water velocity and wind speed were significantly lower in ditch than in reservoir and river, suggesting that they also played important roles in explaining the spatial variability of dissolved CH₄ concentrations and fluxes. These results highlighted a need for more field measurements with wider spatial coverage and finer frequency, which would further improve the reliability of flux estimates for assessing the contribution of agricultural watersheds to the regional and global CH₄ budgets.