Direct emission factors for inventorying NOx emissions from managed soils as part of the IPCC methodology framework

The current inventory framework of the Intergovernmental Panel on Climate Change (IPCC) relies on a single generalized default direct emission factor (EF), which inadequately captures the substantial heterogeneity in nitric oxide (NO) emissions from managed soils and may therefore lead to systematic biases in nitrogen oxides (NOx) inventories across vegetation types and environmental conditions. To address this limitation, and given that soil-emitted NOx is predominantly released as NO, we synthesized global field observations of direct EFs for NO to develop an updated methodological framework for Tier 1- and Tier 2-based inventories of national and global direct NOx emissions from managed soils. For Tier 1, our synthesis yielded 0.49% (95% confidence interval: 0.41%–0.58%), 1.66% (1.17%–2.15%), 0.10% (0.05%–0.17%), and 0.11% (0.06%–0.18%) as aggregated EFs for managed soils (I) growing upland vegetation types other than tea plantations (EF1,O(NOx)), (II) utilized for tea plantations (EF1,Tea(NOx)), (III) cultivated with flooded rice (EF1,FR(NOx)), and (IV) receiving urine and dung deposition by grazing animals (EF3(NOx)), respectively. Significant differences in direct EFs were observed among major cropping systems, with the highest, intermediate and lowest values occurring in types II, I and III, respectively (p = 0.001). For type I, soil organic carbon (SOC) content was identified as the most robust factor regulating direct EFs of NO (p = 0.001). Accordingly, for one Tier 2 option, this type was further disaggregated into two SOC-based subcategories, with direct EFs of 0.31% (0.25%–0.37%) for SOC ≤ 1% (EF1,O,SOC≤1%(NOx)), and 0.62% (0.49%–0.75%) for SOC > 1% (EF1,O,SOC>1%(NOx)). These newly developed direct EFs would enable more accurate inventories of NOx emissions from managed soils and better support targeted mitigation strategies.