Global variation of adjusted N2O EFs

Title: Legacy effects cause systematic underestimation of N₂O emission factorsAbstract: Agricultural soils account for ~50% of global anthropogenic nitrous oxide (N₂O) emissions, which are largely derived from nitrogen (N) fertilizer input. Global N₂O emission factors (EFs) estimated from IPCC Tier 1 methodologies, which are mainly based on field measurements, are 2-5 times lower than those from top-down models. The reason for this inconsistency is unclear, causing uncertainty in global estimates of N₂O fluxes and hindering the development of sound mitigation policies. Here, we show through data synthesis and a series of experiments, that EFs measured in field experiments increase exponentially with measurement duration. Our results can be explained by the legacy effect of N input; historic N addition increases soil N availability, lowers soil pH, and increases the abundance of N₂O producing microorganisms in control plots. Accounting for this legacy effect and without considering other effects, we estimate that global EFs and annual fertilizer-induced N₂O emissions of cropland are 1.7% and 1.8 Tg N₂O-N, respectively, both 89% higher than IPCC estimates. Our findings narrow the gap between bottom-up and top-down estimates of EFs, emphasize the importance of long-term well-controlled experiments for N₂O emission estimates, and underline the need for N₂O emission mitigation practices.Data: Global variation of adjusted N₂O EFs based on EFs from IPCC (2019) and Cui et al. (2021). Please open it in MATLAB.