Identifying nitrogen source and transport characteristics of the urban estuaries and gate-controlled rivers in northern Taihu Lake, China

Revealing nitrogen source and transport mechanisms are essential to achieving optimal nitrogen management in urban estuaries and gate-controlled rivers. However, in plains river networks, gate and/or dam construction make the nitrogen sources difficult to trace. In this study, seasonal water samples from typical urban estuaries and gate-controlled rivers in northern Taihu Lake were analyzed using statistical analysis and stable isotope analysis in R (SIAR). The results show that the concentrations of various nitrogen forms showed obvious spatial-temporal distribution characteristics by multiple sources. Specifically, the nitrogen concentration was lower during the wet season than the dry season (T = 6.02, p < 0.01). Gate construction had a dual effect on the water environment, because it changed both the nitrogen transport processes and the nitrogen enrichment risk between the river side and lake side. The 818O-NO3- and 815N-NO3- values were sensitive to anthropogenic activity intensity and urbanization process, showing the following order of 815N-NO3- values: Countryside > Suburbs > Downtown, and Construction Land > Forest-grassland > Farmland. While, a contrary trend was observed among the variations in 818O-NO3- values. According to the SIAR model, sewage/manure, fertilizer, and sediment nitrogen (sediment N) were the main nitrate sources, accounting for 42.68%, 23.79%, and 23.71%, respectively. The proportion of urban nitrogen discharge has surpassed that of agricultural sources, becoming the largest nitrogen source in the waterbody. Under the coupling of meteorological conditions and estuary shape, the endogenous nitrogen released from sediment became the second largest contributor in some estuaries. These findings suggest that it is necessary to strengthen the control of urban exogenous nitrogen input and sediment nitrogen endogenous release to enable future estuarine nitrogen management and habitat restoration.