Stable isotopic characterization and sources of ammonium in wet deposition at the Danjiangkou Reservoir

Abstract

Ammonium nitrogen (NH_x) in atmospheric wet deposition is a primary external nitrogen source for reservoir-type water sources, and identifying its sources is crucial for controlling nitrogen pollution in water. This study aimed to examine the NH_x characteristics and sources in wet deposition in the Xichuan reservoir area of the Danjiangkou Reservoir, the South-to-North Water Diversion Project (SNWDP) water source, from September 2019 to August 2020. Major inorganic nitrogen concentrations in precipitation and ammonia-nitrogen isotope (δ¹⁵N–NH₄⁺) values were measured, and the sources were analyzed using the Bayesian mixing model. The results showed that NH₄⁺ was the main inorganic nitrogen form in wet deposition, with a concentration ranged of 0.16–5.26 mg L⁻¹. The δ¹⁵N–NH₄⁺ values ranged from −14.7‰ to +6.3‰, with significant isotopic effects from agricultural sources and climatic conditions, showing higher values in summer and lower values in winter. The release of ammonia (NH₃) from agricultural sources was the primary NH_x source (58.0%–76.6%), with fertilizer application being the most significant contributor (32.1%–46.6%). Seasonally, the relative contribution of agricultural sources to NH₄⁺ in wet deposition was higher in autumn and winter. Spatially, agricultural activities significantly impacted atmospheric nitrogen accumulation across the reservoir area. This study quantified NH_x sources in wet deposition, providing isotopic evidence and scientific references for nitrogen control measures and atmospheric nitrogen cycling studies.