Global variation of adjusted N2O EFs
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Title: Legacy effects cause systematic underestimation of N<sub>2</sub>O emission factorsAbstract: Agricultural soils account for ~50% of global anthropogenic nitrous oxide (N<sub>2</sub>O) emissions, which are largely derived from nitrogen (N) fertilizer input. Global N<sub>2</sub>O 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 N<sub>2</sub>O 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 N<sub>2</sub>O producing microorganisms in control plots. Accounting for this legacy effect and without considering other effects, we estimate that global EFs and annual fertilizer-induced N<sub>2</sub>O emissions of cropland are 1.7% and 1.8 Tg N<sub>2</sub>O-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 N<sub>2</sub>O emission estimates, and underline the need for N<sub>2</sub>O emission mitigation practices.Data: Global variation of adjusted N<sub>2</sub>O EFs based on EFs from IPCC (2019) and Cui et al. (2021). Please open it in MATLAB.
标题：遗留效应导致一氧化二氮（N₂O）排放因子的系统性低估 摘要：农业土壤约占全球人为一氧化二氮（N₂O）排放总量的50%，此类排放主要源于氮肥（N）投入。基于主要依靠田间实测的政府间气候变化专门委员会（IPCC）第一层（Tier 1）方法学估算得到的全球N₂O排放因子（EFs），较自上而下模型的估算结果低2~5倍。这种不一致的成因尚不明确，这不仅给全球N₂O通量估算带来了不确定性，也阻碍了合理减排政策的制定。本研究通过数据综合分析与一系列实验证实：田间实验测得的排放因子会随观测时长呈指数级增长。该结果可通过氮素投入的遗留效应得到解释：历史氮素添加会提升土壤有效氮含量、降低土壤pH值，并增加对照样地中产N₂O微生物的丰度。在仅考虑该遗留效应、不纳入其他影响因素的前提下，我们估算得到全球农田排放因子与氮肥诱导的年N₂O排放量分别为1.7%与1.8 Tg N₂O-N，较IPCC的估算结果均高出89%。本研究的发现缩小了排放因子自下而上与自上而下估算结果之间的差距，强调了长期受控良好的实验对于N₂O排放估算的重要性，并明确了推行N₂O减排措施的必要性。 数据集：基于政府间气候变化专门委员会（IPCC，2019年）与崔等（Cui et al., 2021）的排放因子修正得到的全球N₂O排放因子变异情况，请在MATLAB中打开该数据集。
