Agricultural Ditches are Hotspots of Greenhouse Gas Emissions Controlled by Nutrient Input

Agricultural ditches are pervasive in agricultural areas and are potential greenhouse gas (GHG) hotspots, since they directly receive abundant nutrients from neighboring farmlands. However, few studies measure GHG concentrations or fluxes in this particular waterbody, likely resulting in underestimations of GHG emissions from agricultural regions. Here we conducted a one-year field study to investigate the GHG concentrations and fluxes from typical agricultural ditch systems, which include main ditches (MD), branch ditches (BD), collector ditches (CD), and field ditches (FD), in an irrigation district located in the North China Plain. The results showed that almost all the ditches were large GHG sources. The mean fluxes were 333 μmol m^-2 h^-1 for CH₄, 7.1 mmol m^-2 h^-1 for CO₂, and 2.4 μmol m^-2 h^-1 for N₂O, which were approximately 12, 5, and 2 times higher, respectively, than that in the river connecting to the ditch systems. Nutrient input was the primary driver stimulating GHG production and emissions, resulting in GHG concentrations and fluxes increasing from the river to MDs, BDs, and then CDs as the ditch systems approached farmlands and potentially received more nutrients. Despite FDs being directly connected to farmlands, their GHG concentrations and fluxes were lower due to seasonal drying and occasional drainage. The ditches covered approximately 3.3% of the 312 km² farmland area in the study district and the total GHG emission from the ditches in this area was estimated to be 26.6 Gg CO₂-eq yr^-1, with 17.5 Gg CO₂, 0.27 Gg CH₄, and 0.006 Gg N₂O emitted annually. Overall, this study demonstrated that agricultural ditches were hotspots of GHG emissions, and future GHG estimations should incorporate this ubiquitous but underrepresented waterbody.