Improving Agricultural Nitrogen Use Efficiency to Reduce Air Pollution in China

Chinese agriculture has long been characterized by low nitrogen use efficiency (NUE) associated with substantial ammonia (NH₃) loss, which contributes significantly to fine particulate matter (PM_2.5) pollution. However, the knowledge gaps in the spatiotemporal patterns of NH₃ emissions and the states of nitrogen management of agricultural systems render it challenging to evaluate the effectiveness of different mitigation strategies and policies. Here we explored the NH₃ mitigation potential of various strategies and its subsequent effects on PM_2.5 pollution, and their effectiveness in improving NUE of Chinese agricultural systems. We developed and used a nitrogen flow model for evaluating NUE of different crop and livestock types at a provincial scale in China. We then used the bottom-up NH₃ estimates to drive an air quality model (GEOS-Chem High Performance, GCHP) to provide an integrated assessment of four improved nitrogen management scenarios: improving NUE of crop systems (NUE-C), increasing organic fertilizer use (OUR), improving NUE of livestock systems (NUE-L) and combined measures (COMB). The total agricultural NH₃ emission of China was estimated to be 11.2 Tg NH₃ in 2017, of which 46.24% and 53.76% are attributable to fertilizer use and livestock animal waste, respectively, and emission hotspots can be identified in the North China Plain. Our results show that grain crops have higher NUE than fruits and vegetables, while high livestock NUE can be found in pork and poultry, and NUE for the entire crop and livestock systems are both better in Northeast China than the rest of China. We also found that agricultural NH₃ emissions can be reduced from 11.2 Tg to 9.1 Tg, 9.3 Tg, 9.9 Tg and 6.8 Tg, and consequently annual population-weighted PM_2.5 reductions are estimated to be 1.8 µg m^–3, 1.6 µg m^–3, 1.3 µg m^–3 and 4.1 µg m^–3 under NUE-C, OUR, NUE-L and COMB scenarios, respectively. Our results are expected to provide decision support policy making concerning agricultural NH₃ emissions.