Integrated environmental thresholds for nitrogen in China

The environmental problems caused by the overuse of nitrogen (N) fertilizer is well recognized. However, the complexity of the N cycle and its multiple pollution pathways have hindered the quantification of a safe operating space for N fertilizer use in China. We used a modeling approach and environmental thresholds for N deposition, N concentrations in surface water and groundwater, as well as for ammonia (NH₃) volatilization and nitrous oxide (N₂O) emissions to assess spatial patterns and quantify N use mitigation goals for different regions in China.

At the national scale, the results indicate that the safe operating space for N use can be achieved if total N deposition is reduced to 5.2 Tg N yr^-1, total loading of N to surface water and groundwater are reduced to 5.4 and 13.9 Tg N yr^-1, respectively, and total NH₃ volatilization and N₂O emissions are below 5.3 and 0.6 Tg N yr^-1, respectively. Meeting these thresholds would require reductions of approx. 22%, 44%, 11%, 48%, and 30%, respectively. In total, this amounts to a reduction of 12.5 Tg N yr^-1, or a 29% decrease from current levels of N inputs to the environment. Specifically, central China and southern China require higher emission reductions to meet the thresholds, particularly in provinces such as Henan, Shandong, and Sichuan. This study is the first to integrate multiple N indicators to determine a national reduction target for China. This approach provides a scientific basis for improving N management, mitigating its environmental impacts and identifying regional “low-hanging fruits” where targeted reductions could yield the greatest environmental benefits.