Based on the analysis of stable nitrogen isotope (δ~(15)N), 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.δ~(15)N of dissolved nitrate (NO_3-) 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 ofδ~(15)N 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 ofδ~(15)N 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 ofδ~(15)N in study of estuarine nitrogen cycle.Theδ~(15)N values of dissolved nitrate (δ~(15)N-NO_3-) 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 ofδ~(15)N-NO_3- 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 theδ~(15)N-NO_3- signatures in different geographic regions. In the inner estuary,δ~(15)N-NO_3- 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 ofδ~(15)N-NO_3- were investigated, indicating complex processing of NO_3-. In the adjacent sea, seasonal variations ofδ~(15)N-NO_3- were consistent with those of nitrate uptake rates in phytoplankton as a result of biological processing.At the same time, theδ~(15)N values of SPM (δ~(15)Np) 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 inδ~(15)Np 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,δ~(15)Np 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 inδ~(15)Np during four seasons. In adjacent marine sea, the influence of terrigenous inputs weakened andδ~(15)Np 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 betweenδ~(15)N-NO_3- andδ~(15)Np reflected some biogeochemical processing and environmental implications. In general,δ~(15)N-NO_3- was slightly lower thanδ~(15)Np. The nitrogen fractionation (ε) between them was positive in average. The lowestδ~(15)N-NO_3- andδ~(15)Np were both investigated in February, indicating consistent influence from external nitrogen inputs and weakened biogeochemical processing in waters. In May, theδ~(15)N-NO_3- andδ~(15)Np values were close to those in November, respectively. And the fractionation (ε) ofδ~(15)N-NO_3- andδ~(15)Np 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 highδ~(15)N-NO_3- and lowδ~(15)Np was most positive in four seasons, probably caused by intense decomposition of particulate matters.Obvious spatial and temporal variations were also observed inδ~(15)N of dissolved nitrate and SPM from the Yangtze River mainstream in 2006. The seasonal variations ofδ~(15)N-NO_3- andδ~(15)Np were similar, with highδ~(15)N values in wet season (in May and August) and lowδ~(15)N values in dry season (in February and November). There was obvious positive correlation betweenδ~(15)N-NO_3- andδ~(15)Np. The uniform spatial distribution ofδ~(15)N-NO_3- andδ~(15)Np 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 theδ~(15)N values of dissolved nitrate and SPM increased with human activity, industrialization and urbanization.