Asymmetry of safeguarding regional air and water nitrogen boundaries in China

Human activities have significantly disrupted the global nitrogen cycle, positioning it as one of the most severely surpassed planetary boundaries. As the country with the largest nitrogen flux, China faces numerous environmental challenges due to excessive losses of reactive nitrogen (N_r) to both air and water from various sources. By quantifying the regional nitrogen boundaries for air and water at the county level, we found that the aggregated regional safe boundaries in China for the atmospheric release of N_r, nitrogen runoff to surface water and leaching to groundwater are 14.6, 5.2 and 4.8 million tonnes per year, respectively. In 2020, the cumulative N_r losses exceeded these boundaries by 54%, 262% and 258%, respectively. Implementing cross-system technical mitigation measures could potentially halve the total N_r losses to both air and water, yielding benefits that are ∼2.5 times greater than the net implementation costs. Despite most counties being capable of meeting the emission boundary for the atmospheric release of N_r after abatement, the boundaries for surface water and groundwater remain exceeded in over half of the counties. This highlights a significant asymmetry in nitrogen-pollution control between air and water, further necessitating socioeconomic transformations to effectively address the persistent issue of water pollution in China.