# Asymmetry of safeguarding regional air and water nitrogen boundaries in China

## TL;DR

This study shows that nitrogen pollution in China is much worse in water than in air, highlighting the need for broader changes to tackle water pollution.

## Contribution

The study introduces a county-level assessment of nitrogen boundaries in China, revealing an air-water pollution asymmetry.

## Key findings

- China's Nr losses exceeded safe boundaries by 54% in air, 262% in surface water, and 258% in groundwater in 2020.
- Mitigation measures could halve Nr losses, with benefits 2.5 times greater than implementation costs.
- Over half of Chinese counties still exceed water nitrogen boundaries despite air pollution improvements.

## Abstract

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 (Nr) 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 Nr, 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 Nr losses exceeded these boundaries by 54%, 262% and 258%, respectively. Implementing cross-system technical mitigation measures could potentially halve the total Nr 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 Nr 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.

This study provides a whole-system, county-level assessment of nitrogen pollution exceedance in China. It revealed a pronounced air-water asymmetry, with water pollution substantially harder to bring within safe limits than atmospheric emissions, underscoring the need to couple mitigation with broader socioeconomic transformations to address persistent water pollution.