Reducing atmospheric ammonia concentrations in typical farming systems surrounding lakes is essential for lake water...

Atmospheric ammonia (NH₃) and its deposition are primary contributors to lake eutrophication. However, long-term spatiotemporal observations of NH₃, particulate and wet ammonium (NH₄⁺), their isotopic signatures, and source contributions in contrasting cropland systems near plateau lakes remain limited. A two-year continuous field observation was conducted in two typical cropland systems adjacent to Erhai Lake, an alpine fault lake in Yunnan province of China: vegetable system (V) and rice-rapeseed rotation system (R). We monitored concentrations of gaseous NH₃, particulate and wet NH₄⁺, their δ¹⁵N values, and quantified source contributions using a Bayesian isotope mixing model while considering nitrogen isotope fractionation. The average NH₃ concentrations in the V system were 3.59 times higher than those in the R system, with pronounced peaks observed in summer and autumn. In contrast, particulate NH₄⁺ concentrations were higher in spring and winter (0.84 and 0.77 μg m^-3 in the V and R systems, respectively). Mean wet NH₄⁺ concentrations were 0.54 and 0.77 mg L^-1 in the V and R systems, respectively. Fertilizer and livestock manure together contributed 74.3% and 83.1% to gaseous NH₃, 65.6% and 59.3% to particulate NH₄⁺, and 96.2% and 84.2% to wet NH₄⁺ in the V and R systems, respectively. Fossil fuel and vehicle exhaust had relatively high contributions to particulate NH₄⁺. Emission sources and meteorological conditions together influenced spatio-temporal NH₃ concentrations across different cropping systems. This study provides evidence for NH_x source control and deposition mitigation around plateau lakes, supporting agricultural NH₃ abatement strategies to protect water quality and mitigate lake eutrophication.