Patterns of nitrogen concentrations and their controls in two southern China urban river ecosystems

摘要

Urbanization influences both inorganic and organic nitrogen (N) forms in aquatic ecosystems, but most urban studies focus on these forms in isolation. This can have significant effects on N biogeochemical cycle, N budgets and N pollution abatement measures. In this regard, we conducted a comprehensive seasonal study to compare the patterns and controls of surface water inorganic and organic N forms between two contrasting Fuzhou river ecosystems in southern China. Surface waters of the urban Bailongjiang River (BJR) had significantly higher dissolved inorganic N (DIN; p ?0.001) and dissolved organic N (DON; p =?0.006) as well as total dissolved N (TDN; p ?0.001) than those of the suburban Wulongjiang River (WJR). This suggests that the BJR surface waters are currently experiencing elevated levels of anthropogenically driven N pollution, which can adversely affect the aquatic and human health. Surface waters during the summer were generally characterized by much lower N concentrations resulting from increased precipitation-driven dilution effects and temperature-induced biological activities than those across the other seasons ( p ?0.05 in most cases). The ammonium-N (NHsub4/subsup+/sup-N)/nitrate-N (NOsub3/subsup?/sup-N) ratios were 1 whereas the DIN/DON ratios were 1, indicating the dominance of NOsub3/subsup?/sup-N over NHsub4/subsup+/sup-N in surface water DIN and DIN over DON in surface water TDN across rivers and seasons. In addition, the contributions of NHsub4/subsup+/sup-N, NOsub3/subsup?/sup-N and DON to the TDN followed the NOsub3/subsup?/sup-N??DON??NHsub4/subsup+/sup-N pattern in the BJR and WJR surface waters. During the entire course of the study, the concentrations of different surface water N forms were strongly positively correlated with each other ( p ?0.05 in all cases), indicating that these investigated N forms were both influenced by urbanization-driven anthropogenic activities. Surface water N concentrations increased significantly with increasing precipitation and soil N concentrations (all p ?0.05). This suggests that increasing atmospheric N deposition and soil N pollution due to rapid urbanization will have dramatic consequences on surface water eutrophication. This study emphasizes that inorganic and organic N should be collectively examined for sustainable eutrophication abatement as their patterns in urban aquatic ecosystems are not only controlled by complex processes but also interdependent.