Atmospheric nitrogen species distribution under influence of agricultural sources in a Brazilian region

Atmospheric deposition is a key process to improve the understanding of human impacts on the nitrogen biogeochemical cycle and its processes. In order to evaluate the nitrogen atmospheric deposition species we quantified the dissolved inorganic nitrogen (DIN = NH₄⁺ + NO₃^-) and dissolved organic nitrogen (DON) fluxes in Lavras, Minas Gerais, Brazil from May 2018 until April 2019. Our sampling site region in Minas Gerais has a strong influence of local sources, such as fertilizers production and application. Moreover, Lavras has 19% (107 km²) of its total area associated with crops plantation. We collected wet and bulk atmospheric deposition samples (n = 50) and analyzed by Kjeldahl method (Total Dissolved Nitrogen – TDN) and ionic chromatography (Dissolved Inorganic Nitrogen – DIN) analysis, respectively. Overall, our results showed that wet deposition of DON was dominant, accounting for 7.34 kg.ha⁻¹.yr⁻¹, while NH₄⁺ and NO₃^- fluxes were of 1.39 and 3.94 kg.ha⁻¹.yr⁻¹, respectively. In addition, the NO₃^-/NH₄⁺ molar ratio ranged from 1.33 to 33.7 and the pH values ranged from 5.66 to 6.08. The lower values of NO₃^-/NH₄⁺ were associated with an alkaline behavior, suggesting dominance of NH₃ in the atmosphere neutralization processes. We noted that DIN species fluxes, NH₄⁺ and NO₃^-, were concentrated between October 2018 and March 2019, thus less than 14% of NH₄⁺ and NO₃^- annual deposition were observed in the dry period (April to September). This pattern was associated with seasonal rainfall distribution in Lavras, and with the volatilization of NH₃ and NO_x species due to fertilizers applications. Regarding DON annual deposition, we observed that such distribution was also present, whereas in a less pronounced percentage (64% in the wet period), suggesting that different processes also play role in Lavras.