By combining high-resolution emission inventories with systematic measurements of wet and dry deposition, the team determined that annual emissions surpass local deposition by more than 8,200 tons of nitrogen. The analysis shows that ammonia emissions are overwhelmingly driven by agricultural activities, whereas nitrogen oxide emissions originate primarily from the transportation sector.
Reactive nitrogen—including ammonia (NH₃), nitrogen oxides (NOₓ), ammonium, and nitrate—plays a key role in air pollution, climate change, and ecosystem degradation. NH₃ promotes fine particulate matter (PM₂.₅) formation, while NOₓ drives ozone and secondary aerosol production. Excess nitrogen deposition can acidify soils, trigger lake eutrophication, and reduce biodiversity. Although China has significantly reduced NOₓ emissions through industrial and transportation controls, agricultural NH₃ emissions have remained largely unchanged. Previous studies often assessed emissions or deposition separately, leaving uncertainties in overall nitrogen budgets. The Erhai Lake Basin, a sensitive subtropical plateau ecosystem undergoing agricultural reform, provides an ideal case for an integrated source–sink evaluation.
A study (DOI: 10.48130/nc-0025-0018) published in Nitrogen Cycling on 19 January 2026 by Wen Xu's team, China Agricultural University, provides the first integrated source–sink assessment demonstrating that the Erhai Lake Basin is a substantial net exporter of atmospheric reactive nitrogen, offering critical scientific evidence for coordinated agricultural and transportation emission control in fragile plateau lake ecosystems.
Using a bottom-up emission inventory combined with spatial analysis and a nine-site atmospheric monitoring network, the researchers first quantified NH₃ and NOₓ emissions for 2022 and then measured wet and dry nitrogen deposition across the basin in 2023 to construct a complete atmospheric Nr budget. The emission inventory integrated detailed activity data and sector-specific emission factors, while deposition fluxes were derived from measured gaseous concentrations, precipitation chemistry, and inferential dry deposition calculations. Results showed that total Nr emissions reached 10,720.4 t N yr⁻¹, with NOₓ-N (55.6%) slightly exceeding NH₃-N (44.4%). Agriculture overwhelmingly dominated NH₃ emissions (91.7%), split between livestock (48.9%)—mainly dairy cattle, laying hens, and hogs—and fertilizer application (42.8%), primarily from maize, vegetables, fruit trees, soybeans, and rice. In contrast, transportation accounted for 98.2% of NOₓ emissions, driven largely by heavy-duty trucks and passenger vehicles. Spatial mapping revealed concentrated agricultural NH₃ hotspots in northern townships such as Sanying and Zibihu, whereas vehicle-related NOₓ emissions were more evenly distributed around urban and transport hubs. Deposition monitoring showed an annual average total Nr flux of 10.4 ± 1.0 kg N ha⁻¹ yr⁻¹, dominated by reduced nitrogen (68.9%), with dry deposition contributing 58.2% of the total. NH₃ was the principal component of dry flux, while NH₄⁺ dominated precipitation chemistry. Seasonally, deposition peaked in summer and shifted between dry-dominated pathways in winter and spring and wet-dominated pathways in summer and autumn. When emissions were compared with deposition, a pronounced surplus emerged: the basin exported 8,201.2 t N yr⁻¹ overall, including large surpluses of both reduced (3,001.7 t) and oxidized nitrogen (5,420.8 t). These consistent imbalances across sub-regions confirm that the ELB functions as a substantial net atmospheric source of reactive nitrogen.
In conclusion, the Erhai Lake Basin functions as a significant hotspot of reactive nitrogen export, with atmospheric inputs contributing markedly to the lake's total nitrogen burden despite moderate per-area deposition. The large emission surplus highlights the necessity of coordinated mitigation strategies. Effective management must simultaneously reduce agricultural ammonia through improved manure and fertilizer practices while curbing transportation-related NOₓ via cleaner vehicles and stricter standards to prevent offsetting gains and ensure sustainable ecosystem protection.
