Differential patterns of nitrogen and δN in soil and foliar along two urbanized rivers in a subtropical coastal city of southern China.

Urbanization usually pollutes the environment leading to alterations in key biogeochemical cycles. Therefore, understanding its effects on forest nitrogen (N) saturation is becoming increasingly important for addressing N pollution challenges in urban ecosystems. In this study, we compared soil (N availability, net N mineralization, net nitrification, and δN) and foliar (N concentrations and δN) variables in upstream, midstream and downstream forest stands of Bailongjiang River (BJR; more urbanized) and Wulongjiang River (WJR; less urbanized), the two branches of the Minjiang River Estuary. Total soil N, ammonium, nitrate, net N mineralization and nitrification rates, as well as soil δN were significantly higher in BJR compared with WJR forest stands. While no substantial difference in foliar N concentrations was noted between rivers, foliar δN was on average more than 2.5 times higher in BJR than WJR forest stands. Across the study area, foliar δN was positively related to soil δN, which also had positive linear relationships with soil nitrate concentrations, net N mineralization and net nitrification rates. Moreover, all variables except foliar δN and ammonium concentrations showed decreasing patterns in the order: upstream > midstream > downstream along the BJR forest stands. Soil ammonium and foliar values (N concentrations and δN) revealed clear patterns along the WJR, with the former increasing and the latter decreasing from the upstream to downstream forest stands. Our findings indicate an increase in urbanization-induced N inputs from the WJR to BJR and that forest stands along the BJR especially at the upstream have higher N availability and are shifting rapidly towards N saturation state. These results emphasize the need for effective N pollution control in urban environments through sustainable urban planning.