A large net source revealed by the atmospheric reactive nitrogen budget in a subtropical plateau lake basin, southwest China

Quantifying the source–sink budget of atmospheric reactive nitrogen (Nr) is crucial for mitigating Nᵣ pollution in vulnerable lake ecosystems. Here, a comprehensive analysis of the atmospheric Nr budget in the Erhai Lake Basin (ELB) (a subtropical plateau lake in southwestern China) is presented. Emission of ammonia (NH3) and nitrogen oxides (NOx) for 2022 emissions were estimated using detailed activity data and localized emission factors, while dry and wet Nr deposition in 2023 was quantified through a nine-site monitoring network. The one-year difference, necessitated by data availability, does not affect the core conclusions, given the stability of the fundamental emission source structure. Total atmospheric Nr emissions were estimated at 10,720.4 t N yr−1, dominated by NOx-N (55.6%), followed by NH3-N (44.4%). Agriculture was the primary source of NH3-N (91.7%), with nearly equal contributions from livestock (48.9%) and fertilization (42.8%). In contrast, the transportation sector dominated NOx-N emissions (98.2%). Annual Nr deposition averaged 10.4 kg N ha−1 yr−1, predominantly in the reduced form (NHx-N, 68.9%). The net atmospheric Nr budget revealed a substantial surplus of 8,201.2 t N yr−1, unequivocally identifying the ELB as a significant net source. The present results underscore the imperative for integrated mitigation strategies that concurrently address emissions from agricultural and vehicles to alleviate regional Nᵣ pollution.