Increasing importance of nitrate-nitrogen and organic nitrogen concentrations in bulk and throughfall precipitation across urban forests in southern China

Abstract

Atmospheric nitrogen (N) deposition is an increasingly serious threat to forest ecosystems requiring urgent global actions. While it is well-known that developing countries are experiencing rapid urbanization, an understanding of its effects on N composition and sources in atmospheric deposition is only beginning to emerge. We measured dissolved inorganic N (DIN), dissolved organic N (DON) and total dissolved N (TDN) concentrations in bulk precipitation (BP) and throughfall precipitation (TP) across four seasons in forest ecosystems along the more urbanized river (Bailongjiang; BJR) and the less urbanized river (Wulongjiang; WJR) in Fuzhou coastal city. Concentrations of all N forms were greatly enhanced in BJR forests than in WJR forests, suggesting increased anthropogenic N pollution in forests along the BJR that, in turn, can cause serious N cycle perturbations. While precipitation N concentrations over the BJR forests were primarily influenced by pollutants from fossil fuel combustion activities (ammonium-nitrogen; NH4+-N/nitrate-nitrogen; NO3−-N = 0.77), those over the WJR forests were collectively influenced by pollutants from fossil fuel combustion and agricultural activities (NH4+-N/NO3−-N = 1.14). The DON contributed 22–38% of N to the TDN across seasons and rivers and was positively related to other N forms, indicating that DON is steadily becoming a vital component in atmospheric N deposition especially in anthropogenically N-polluted atmospheres. Summer was characterized by precipitation with lower N concentrations, highlighting increased dilution effect and decreased anthropogenic N emissions. Nitrogen concentrations were more elevated in TP than in BP, suggesting that tree canopies augment atmospheric N deposition to forest soils. Generally, soil and foliar TN and δ15N were positively related to precipitation TDN, indicating that increased atmospheric N deposition enhances forest ecosystem N availability. Our results demonstrate that urban areas particularly in developing countries are not only NHx but also NOx emission and deposition hotspots and emphasize the necessity of including them in future global N budgets and N pollution abatement initiatives.