Study On The Variability Of Nitrogen Isotope In The Yangtze River (Changjiang) Estuary, China And Its Environmental Implications

Based on the analysis of stable nitrogen isotope (δ¹⁵N), variability of nitrogen isotope in the Yangtze River (Changjiang) estuary and its environmental implications were studied. The comprehensive preparation method for stable nitrogen isotopic analysis in waters was given.δ¹⁵N of dissolved nitrate (NO₃-) and suspended particulate matters (SPM) in surface water of the Yangtze River mainstream and its estuary were analyzed by using the preparation method in 2006. According to the variations ofδ¹⁵N in different seasons and geographical regions, the biogeochemical processing of nitrogen transformation was studied and its environmental implications were also revealed. In this sense, the mechanism of nitrogen cycle was explored in the Yangtze River estuary.The main results are as follows:The preparation method of water samples is critical to identify both inaccurate and accurate analysis ofδ¹⁵N in seawater. The method varies with different nitrogen form and water bodies. In this paper, based on the traditional distillation, a suitable method for the preparation of dissolved nitrate samples in seawater for nitrogen isotopic analysis was studied and improved. Some tests of the method were conducted and good effect on analysis was found. Additionally, both the preparation method of dissolved nitrate in freshwater and the method of nitrogen isotopic analysis for SPM were studied. The comprehensive preparation method for stable nitrogen isotopic analysis in waters was given, which can supply basic information for wide application ofδ¹⁵N in study of estuarine nitrogen cycle.Theδ¹⁵N values of dissolved nitrate (δ¹⁵N-NO₃-) in surface water of the Yangtze River estuary were analyzed in February, May, August and November of 2006 by using above method. It was indicated that the distribution ofδ¹⁵N-NO₃- varied with seasons and geographic regions, with an average of 3.5‰(ranging from 0.4‰to 6.5‰). Different physical and biogeochemical processes affected theδ¹⁵N-NO₃- signatures in different geographic regions. In the inner estuary,δ¹⁵N-NO₃- was affected mainly by riverine input from the Yangtze River with single nitrogen source. In the Turbidity Maximum zone, irregular distribution and non-conservative mixing behaviour ofδ¹⁵N-NO₃- were investigated, indicating complex processing of NO₃-. In the adjacent sea, seasonal variations ofδ¹⁵N-NO₃- were consistent with those of nitrate uptake rates in phytoplankton as a result of biological processing.At the same time, theδ¹⁵N values of SPM (δ¹⁵Np) in surface water of the Yangtze River estuary were studied in February, May, August and November of 2006. The spatial and temporal variations were also observed inδ¹⁵Np with a range between 0.6‰and 8.2‰, as the result of different influence of terrigenous inputs and nitrogen biogeochemical processing. In the inner estuary,δ¹⁵Np was affected mainly by riverine inputs from the Yangtze River. In the Turbidity Maximum zone, SPOM was affected by microbial degradation obviously, indicating the control of the decomposition processing of SPOM inδ¹⁵Np during four seasons. In adjacent marine sea, the influence of terrigenous inputs weakened andδ¹⁵Np was controlled by inorganic nitrogen assimilation by phytoplankton and the decomposition of SPOM.The important mutual transformation of dissolved nitrate and SPM was found in the Yangtze River estuary. The variations and correlations betweenδ¹⁵N-NO₃- andδ¹⁵Np reflected some biogeochemical processing and environmental implications. In general,δ¹⁵N-NO₃- was slightly lower thanδ¹⁵Np. The nitrogen fractionation (ε) between them was positive in average. The lowestδ¹⁵N-NO₃- andδ¹⁵Np were both investigated in February, indicating consistent influence from external nitrogen inputs and weakened biogeochemical processing in waters. In May, theδ¹⁵N-NO₃- andδ¹⁵Np values were close to those in November, respectively. And the fractionation (ε) ofδ¹⁵N-NO₃- andδ¹⁵Np was seriously negative in general. It was shown that the dissolved nitrate and SPM were probably affected by similar influence from external inputs or biological assimilation. In August, the fractionation (ε) of highδ¹⁵N-NO₃- and lowδ¹⁵Np was most positive in four seasons, probably caused by intense decomposition of particulate matters.Obvious spatial and temporal variations were also observed inδ¹⁵N of dissolved nitrate and SPM from the Yangtze River mainstream in 2006. The seasonal variations ofδ¹⁵N-NO₃- andδ¹⁵Np were similar, with highδ¹⁵N values in wet season (in May and August) and lowδ¹⁵N values in dry season (in February and November). There was obvious positive correlation betweenδ¹⁵N-NO₃- andδ¹⁵Np. The uniform spatial distribution ofδ¹⁵N-NO₃- andδ¹⁵Np was also found, with an increase from the upper reaches to the lower reaches of the Yangtze River. It was considered that dissolved nitrate and SPM were influenced by similar nitrogen inputs. In the upper reaches, the main nitrogen inputs were from atmosphere deposition and agricultural sources. In the middle and lower reaches, nitrogen sources varied and theδ¹⁵N values of dissolved nitrate and SPM increased with human activity, industrialization and urbanization.