Legacy N2O emissions overlooked and crop-specific mitigation solutions

Agricultural soil nitrous oxide (N2O) emissions generally derive from three sources, namely, N fertilizer-induced seasonal direct emissions and post-seasonal legacy emissions, and background emissions. While most studies have concentrated on fertilizer-induced direct N2O emission factors (EFD) in staple crops, they did not account for N2O emissions induced by the legacy effects of historical N fertilization. Here, we synthesized global long-term field data (≥ 3 years) to map crop-specific legacy N2O emission factors (EFL). The global mean value of EFL was 0.09%, 0.17%, and 0.04% for wheat, maize, and rice production, respectively. Legacy N2O emissions from global wheat, maize, and rice were estimated to be 0.02, 0.04, and 0.01 Tg N2O-N season-1, accounting for 7.3%, 12.6%, and 5.6% of N fertilizer-induced emissions, respectively. Fertilizer-induced direct and legacy N2O emissions from three staple crops totaled 0.87 Tg N2O-N season-1, ~130% higher than IPCC tier1 estimates. Background N2O emissions contributed 33.1% of the fertilizer-induced emissions from staple crops. Crop-specific mitigation solutions through improving N management could reduce global N2O emissions from three staple crops by 0.47 Mt, increase crop production by 362.8 Mt, and potentially feed an additional 8 million people by 2100. Our findings challenge previous incomplete accounting of agricultural N2O emissions and highlight the importance to incorporate legacy effects in emission inventories and development of mitigation strategies for climate-resilient agriculture.

农业土壤一氧化二氮（nitrous oxide, N₂O）排放主要来源于三大途径：氮肥诱导的季节性直接排放、季后遗留排放以及本底排放。当前多数研究聚焦于主粮作物的氮肥诱导直接N₂O排放系数（EFD），却未考虑历史氮肥施用遗留效应所引发的N₂O排放。本研究整合了全球时长≥3年的长期田间试验数据集，构建了作物特异性遗留N₂O排放系数（EFL）的全球分布图谱。小麦、玉米、水稻生产对应的EFL全球均值分别为0.09%、0.17%与0.04%。全球小麦、玉米、水稻的遗留N₂O排放估算值分别为0.02、0.04与0.01太克氧化亚氮-氮（Tg N₂O-N）每季，分别占氮肥诱导排放总量的7.3%、12.6%与5.6%。三大主粮作物的氮肥诱导直接与遗留N₂O排放总计达0.87太克氧化亚氮-氮（Tg N₂O-N）每季，较政府间气候变化专门委员会（Intergovernmental Panel on Climate Change, IPCC）第一层级估算值高出约130%。本底N₂O排放占主粮作物氮肥诱导排放总量的33.1%。通过优化氮肥管理制定的作物特异性减排方案，可在2100年前将全球三大主粮作物的N₂O排放减少0.47百万吨（Mt），并提升作物产量362.8百万吨，额外养活约800万人口。本研究结果挑战了此前农业N₂O排放核算不完整的现状，强调了将遗留效应纳入排放清单以及制定气候韧性农业减排策略的重要性。