Improved accuracy and reduced uncertainty in greenhouse gas inventories by refining the IPCC emission factor for direct N(2)O emissions from nitrogen inputs to managed soils

Most national GHG inventories estimating direct N(2)O emissions from managed soils rely on a default Tier 1 emission factor (EF(1)) amounting to 1% of nitrogen inputs. Recent research has, however, demonstrated the potential for refining the EF(1) considering variables that are readily available at national scales. Building on existing reviews, we produced a large dataset (n = 848) enriched in dry and low latitude tropical climate observations as compared to former global efforts and disaggregated the EF(1) according to most meaningful controlling factors. Using spatially explicit N fertilizer and manure inputs, we also investigated the implications of using the EF(1) developed as part of this research and adopted by the 2019 IPCC refinement report. Our results demonstrated that climate is a major driver of emission, with an EF(1) three times higher in wet climates (0.014, 95% CI 0.011–0.017) than in dry climates (0.005, 95% CI 0.000–0.011). Likewise, the form of the fertilizer markedly modulated the EF(1) in wet climates, where the EF(1) for synthetic and mixed forms (0.016, 95% CI 0.013–0.019) was also almost three times larger than the EF(1) for organic forms (0.006; 95% CI 0.001–0.011). Other factors such as land cover and soil texture, C content, and pH were also important regulators of the EF(1). The uncertainty associated with the disaggregated EF(1) was considerably reduced as compared to the range in the 2006 IPCC guidelines. Compared to estimates from the 2006 IPCC EF(1), emissions based on the 2019 IPCC EF(1) range from 15% to 46% lower in countries dominated by dry climates to 7%–37% higher in countries with wet climates and high synthetic N fertilizer consumption. The adoption of the 2019 IPCC EF(1) will allow parties to improve the accuracy of emissions’ inventories and to better target areas for implementing mitigation strategies.