IPCC Emission Factor\nOverestimates N<sub>2</sub>O\nEmissions from Agricultural Ditches

Agricultural ditches emit disproportionate amounts of\nnitrous oxide\n(N<sub>2</sub>O), but their contributions to regional or global N<sub>2</sub>O emissions remain unclear due to limited data. The Intergovernmental\nPanel on Climate Change (IPCC) recommends using emission factors (EFs)\nto estimate indirect N<sub>2</sub>O emission, but the EF for ditches\n(EF<sub>5g</sub>) is categorized as groundwater, which potentially\nintroduces a significant bias. This study conducted a regional-scale\ncampaign in the North China Plain, one of the world’s most\nintensive agricultural regions, and calculated the EF<sub>5g</sub> values from agricultural ditches by the concentration method (N<sub>2</sub>O–N/NO<sub>3</sub><sup>–</sup>–N). The\nresults found that the regional-scale mean EF<sub>5g</sub> value (0.0028)\nwas less than half of the IPCC default value (0.006), illustrating\nthat the current IPCC methodology significantly overestimates N<sub>2</sub>O emissions from agricultural ditches. Despite the relatively\nsmall EF<sub>5g</sub> values, agricultural ditches exhibited a high\nmean N<sub>2</sub>O concentration (3.36 μg L<sup>–1</sup>) and a large regional emission (1.14 ± 0.86 Gg N<sub>2</sub>O–N yr<sup>–1</sup>), which is equal to 3.8 ±\n2.9% of direct N<sub>2</sub>O emission from the croplands in the North\nChina Plain. Since ditches are ubiquitous in agricultural regions\nand are likely to expand under climate change, refining EF<sub>5g</sub> is crucial to accurately assess their contribution to global N<sub>2</sub>O budgets.