| The estuarine and coastal tidal flat, as a typical transitional zone between land and ocean, is a multifunctional and complex ecosystem with special ecological values and potential resources. Due to the interaction of land and ocean, the estuarine and coastal tidal flat is characterized by intense hydrodynamic conditions, frequent sediment transport and material exchange, steep physiochemical gradients and high biodiversity. In addition, the estuarine and coastal tidal . flat is one of important regions where the biogeochemical cycling is very intense within the ecosystems on the earth's surface. The biogeochemical cycling of nutrient has the important ecological significance to sustaining higher productivity and biomass in the estuarine and coastal tidal ecosystem. Therefore, to study the nutrient biogeochemical cycling in the complex ecosystem has recently become one of headline scientific problems in the fields of physical geography and estuarine and coastal environmental geochemistry. In recent years, the important scientific problem on environmental geochcmical processes across the intertidal sediment-water interface and ecological significance in the Yangtze Estuary has been studied, which was funded by the National Natural Science Foundation of China and the Foundation for University Key Teacher by the Ministry of Education. The main findings are as following.(1)Based on the analysis of stable carbon and nitrogen isotopes in the surface tidal flat sediments from the Yangtze estuary, it was found that the ratios of stable carbon and nitrogen isotopes were respectively 29.80% ~ -23. 7% and 1.0% ~ 5.5% in the flood seasons, while they were -27.3% ?-25. 6% and 1.6% ~ 7.7% in the dry seasons respectively,indicating the seasonal distribution of discharges from theYangtze river has significantly affected the seasonal variation of stable carbon and nitrogen isotopes in sedimentary organic matter. In general, the distribution of stable carbon and nitrogen isotopes revealed the mixing inputs of terrigenous and marine organic matter controlled the stable carbon and nitrogen isotope compositions in sedimentary organic matter. However, a series of physical, chemical and biogeochemical processes occurring in the local environments had also the significant influence on the stable carbon and nitrogen isotope compositions in organic matter. In addition, the contribution of terrigenous input to sedimentary organic matter was quantitatively estimated in light of the mixing balance model of stable carbon isotope. It was revealed that the inputs of terrigenous organic matter occupying about 54.85% of total organic mater inputs was slightly higher than marine inputs in the intertidal sediments, and had the apparent seasonal and spatial difference in the study area. Terrigenous inputs far exceeded marine inputs in the freshwater areas from Station Xupu to Bailonggang, and were respectively 73. 67% and 52. 45% of total organic matter inputs in July and February, which was consistent with the seasonal distribution of water discharge and sediment discharge from the Yangtze River. However, terrigenous inputs obviously decreased in the saltwater areas from Station Chaoyang Farm to Luchao, and were 40. 57% and 46.14% of total organic matter inputs in July and February, respectively.(2)The spatial and temporal distribution of nutrients showed that due to the effects of water discharge from the Yangtze Estuary, coastal rivers and industrial and domestic sewage, there was complex spatial and temporal distribution of nutrient near the intertidal sediment-water interface in the Yangtze estuarine and coastal ecosystem. The diffusive fluxes of nutrient across the intertidal sediment-water interface in the Yangtze Estuary were estimated, using Pick's first law. The spatio-temporaldistribution of NHLf-N diffusive flux was very complex which was dominated by NH-N production. NH/-N mainly diffused from the sediments into the overly waters in the Yangtze estuarine and tidal system, indicating sediments was one of potential sources of N...
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