The Yangtze River estuary is one of the largest estuaries in the world and is comprised of four main channels (North Branch, North Passage, North Channel and South Channel) separated by the islands of Chongming, Changxing, Hengsha and Jiuduansha. These islands have been formed by rapid sedimentation. As sedimentation has progressed, there have been associated changes in total nitrogen and inorganic nitrogen in the surface sediments. Island wetland is one of the most important part of the estuary wetland and island sediments are the key “sink" for many nutrients and have a strong purification function, on the one hand; on the other hand, human activity upstream in the catchment leads to complicated migration and transformation of the nutrient elements, so, sometimes, the region is the “source" of nutrients. Little previous research has been completed on nitrogen concentrations in surface sediments of the estuary islands in relation to recent sediment accumulation. This research investigates the variation in concentrations of nitrogen compounds within sediments in relation to the pattern of recent island sedimentation.
Results show: (1) The most recent sediments, such as on Baimaosha, had a total nitrogen concentration in the surface layer of only 30 mg/kg. In contrast, the oldest sediments, on the east beach of Chongming island, had a concentration of 470 mg/kg. (2) The total nitrogen in the surface sediment of Yangtze River estuarine island wetlands decreased gradually with decrease in altitude, the highest concentrations were in the reed zone, 420 mg/kg, and the lowest were around the low water mark of the beach region, with 110 mg/kg. (3) The proportion of the various inorganic nitrogen compounds to total inorganic nitrogen was relatively stable at all sites. Ammonia contributed the highest proportion at 60%, and nitrite was the lowest, at 17%～19%. Ammonia was therefore the main form of inorganic nitrogen in the Yangtze River estuarine island wetlands. So, with the development of the island to land, the content of the total nitrogen and inorganic nitrogen in the surface sediments would increase.