Differential patterns of nitrogen and d 15 N in soil and foliar along two urbanized rivers in a subtropical coastal city of southern China * Abubakari Said Mgelwa a, b, c , Ya-Lin Hu a, * , Jin-Fu Liu a , Qingyan Qiu a , Zheng Liu a , Mbezele Junior Yannick Ngaba a a Forest Ecology & Stable Isotope Research Center, College of Forestry, Fujian Agriculture & Forestry University, Fuzhou, 350002, People's Republic of China b College of Life Science, Fujian Agriculture & Forestry University, Fuzhou, 350002, People's Republic of China c College of Natural Resources Management & Tourism, Mwalimu Julius K. Nyerere University of Agriculture & Technology, P.O. Box 976, Musoma, Tanzania article info Article history: Received 22 June 2018 Received in revised form 16 October 2018 Accepted 17 October 2018 Available online 25 October 2018 Keywords: Biogeochemical cycles Minjiang river estuary Soil N saturation Urbanization Urban ecosystems abstract 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 d 15 N) and foliar (N concentrations and d 15 N) 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 d 15 N were significantly higher in BJR compared with WJR forest stands. While no substantial difference in foliar N concentrations was noted between rivers, foliar d 15 N was on average more than 2.5 times higher in BJR than WJR forest stands. Across the study area, foliar d 15 N was positively related to soil d 15 N, which also had positive linear relationships with soil nitrate concentrations, net N mineralization and net nitrification rates. Moreover, all variables except foliar d 15 N 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 d 15 N) 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. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Urbanization is increasing rapidly worldwide altering nitrogen (N), carbon (C) and other key biogeochemical cycles across eco- systems. As such, studies of anthropogenic N inputs and its impact on the environment are becoming increasingly important. Previous studies along urbanization gradients have reported higher atmo- spheric N deposition in urban areas than in suburban and rural areas (e.g. Kuang et al., 2011; Huang et al., 2012). Because N deposition generally increases soil N availability (Tian et al., 2018), urban and suburban areas are shifting rapidly towards N saturation (Chen et al., 2010), which has many deleterious ecosystem effects including: plant-soil nutrient imbalances and even forest decline (Aber et al., 2003), eutrophication (Diaz and Rosenberg, 2008), floral and faunal loss (Clark et al., 2013) and atmospheric gas im- balances (Templer et al., 2012), with profound economic and public health consequences (Richter et al., 2005; Dodds et al., 2009). All these negative effects make it important and necessary to under- stand the alteration of N saturation status due to urbanization. The natural abundance of 15 N in soils and plants can serve as an integrator of terrestrial ecosystem N cycling and N dynamics (Robinson, 2001). Despite its importance, interpreting values of d 15 N increase especially of soils remains a challenge given the multiple factors that may potentially discriminate against 15 N and * This paper has been recommended for acceptance by Joerg Rinklebe. * Corresponding author. Forest Ecology & Stable Isotope Research CenterCollege of Forestry, Fujian Agriculture & Forestry University, Fuzhou, 350002, People's Republic of China. E-mail address: huyl@iae.ac.cn (Y.-L. Hu). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol https://doi.org/10.1016/j.envpol.2018.10.083 0269-7491/© 2018 Elsevier Ltd. All rights reserved. Environmental Pollution 244 (2019) 907e914