INT-02 Marine Nitrogen Cycle
Compositions, sources, and transformations of nitrogen in a subtropical watershed, southeastern China
Xiaolan Guo* , Guangdong Provincial Key Laboratory of Marine Disaster Prediction and Prevention, Shantou University, Shantou 515063, China
Qian Yu, Guangdong Provincial Key Laboratory of Marine Disaster Prediction and Prevention, Shantou University, Shantou 515063, China
Hongyan Bao, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361002, China
Jr-chuan Huang, Department of Geography, National Taiwan University
Junwen Wu, Guangdong Provincial Key Laboratory of Marine Disaster Prediction and Prevention, Shantou University, Shantou 515063, China
Shuh-Ji Kao, State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University, Xiamen 361002, China;State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Haikou 570228, China
Xiuli Yan, Guangdong Provincial Key Laboratory of Marine Disaster Prediction and Prevention, Shantou University, Shantou 515063, China

It remains challenging to explore nitrogen (N) dynamics in rivers due to complications associated with intensive anthropogenic inputs and active in-stream biogeochemical processes. In this study, we measured monthly change in concentrations (nitrate: NO3−, nitrite: NO2−, ammonium: NH4+ and particular nitrogen: PN) and isotopic compositions of N species (δ15NNO3−, δ18ONO3−, δ15NNO2−, δ15NNH4+, δ15NPN) to unveil associated N transformations, identify N sources and quantify their relative contributions in the North Stream (NS) and West Stream (WS) of the Jiulong River. Based on the typical isotope range of various N sources, we found that NH4+ was mainly from atmospheric deposition (AD) and manure/septic waste (M&S), while NO3− largely sourced from soil N, M&S and fertilizer. PN was highly contributed by terrestrial inputs resulting from soil erosion and wastes, according to the scatter plots of δ15NPN versus C/N ratios. However, the above isotopic compositions of N species may have been also modulated by in-situ biogeochemical processes. The close correlation between δ15NNH4+ and δ15NPN suggested the influence of NH4+ assimilation with an isotope effect of 12.0±4.0‰ for the NS and 18.2±1.8‰ for the WS. Remineralization can be also suggested in the WS by the positive correlation between δ15NPN and C/N (slope=1.5, r2=0.48, p<0.01). Nitrification was common throughout the year in both two streams since the observed values in δ18ONO3− were highly closed to the theorical values calculating from 1/3 δ18OO2 (23.5‰) and 2/3 δ18OH2O (−7.7 to −5.6‰). By considering above mentioned major N sources and processes in the Bayesian isotope mixing model, we estimated the percentage of anthropic wastes (M&S) to be >56.9% for NH4+, 45.6% for NO3− and 66.0% for PN in the WS. By contrast, in the NS, NO3− (55.0%) and PN (40.4%) were largely contributed by soil N erosion, whereas NH4+ mainly sourced from fertilizer and AD. The variable contributions from various N sources and processes between two streams were highly associated with the degree of urbanization and agricultural structure. This study reveals that the simultaneous use of multiple N isotopes is effective to trace N sources and fates under the complex human inferences in the subtropical rivers.