On the Contribution of Atmospheric Nitrogen Deposition to Nitrogen Burden in an Eutrophic Lake in Eastern China

Atmospheric deposition of natural and anthropogenic sourced reactive nitrogen (Nr, mainly including NH₃, NH₄⁺, NO_x, NO₃^- and etc.) has substantial influence on terrestrial and aquatic ecosystems, driving global nutrient imbalances and increasing risks to human health. Although it has been demonstrated that atmospheric Nr deposition has a substantial impact on nitrogen pools in remote and/or sensitive lakes, there is a scarcity of systematic evaluations regarding atmospheric Nr deposition's impact on the nitrogen burden in eutrophic lakes with riverine input as the primary source. Utilizing a regional chemical transport model, combined with observations of riverine nitrogen input, we investigate the contribution of atmospheric Nr deposition to a eutrophic Lake Chaohu in eastern China. The results indicate that riverine total nitrogen (TN) input to the lake was 11553.3 t N yr^-1 and atmospheric TN deposition was 2326.0 t N yr^-1 in the year of 2022. For Nr species which are directly available for the biosphere supporting algae and plant growth, riverine NH₄⁺ input was 1856.1 t N yr^-1 and atmospheric NH_x (NH₃ and NH₄⁺) deposition was 824.5 t N yr^-1. The latter accounts for ~ 1/3 of total NH_x input to the lake. For NO_y (HNO₃ and NO₃^-) species, atmospheric deposition was estimated to also contributes a similar amount to the NH_x species. The results suggest that even in regions with dense human activities with primary riverine N input, atmospheric deposition of Nr could also contribute significantly to the bio-available nitrogen in lake systems, and addressing eutrophication in Lake Chaohu and other eutrophic lakes will also need to consider the reduction of NH₃ and NO_x (i.e., NO + NO₂, the precursor of NO_y) emissions, in addition to the mitigation of riverine N input.