IPCC Emission Factor Overestimates N₂O Emissions from Agricultural Ditches

Agricultural ditches emit disproportionate amounts of nitrous oxide (N₂O), but their contributions to regional or global N₂O emissions remain unclear due to limited data. The Intergovernmental Panel on Climate Change (IPCC) recommends using emission factors (EFs) to estimate indirect N₂O emission, but the EF for ditches (EF_5g) is categorized as groundwater, which potentially introduces a significant bias. This study conducted a regional-scale campaign in the North China Plain, one of the world's most intensive agricultural regions, and calculated the EF_5g values from agricultural ditches by the concentration method (N₂O-N/NO₃^--N). The results found that the regional-scale mean EF_5g value (0.0028) was less than half of the IPCC default value (0.006), illustrating that the current IPCC methodology significantly overestimates N₂O emissions from agricultural ditches. Despite the relatively small EF_5g values, agricultural ditches exhibited a high mean N₂O concentration (3.36 μg L^-1) and a large regional emission (1.14 ± 0.86 Gg N₂O-N yr^-1), which is equal to 3.8 ± 2.9% of direct N₂O emission from the croplands in the North China Plain. Since ditches are ubiquitous in agricultural regions and are likely to expand under climate change, refining EF_5g is crucial to accurately assess their contribution to global N₂O budgets.