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.