| The Yangtze estuarine and coastal tidal flat, influenced by river runoff and tidal cycle, is a typical transitional zone between land and ocean. Due to its special natural geographical environment and typical dynamic process, it is very important to study the Yangtze Estuary in international estuarine and coastal researches. So taking the Yangtze estuarine and coastal tidal flat as a research object, based on its natural environment, the temporal and spatial distribution of nutrient nitrogen, the early diagenesis processes of core sediments have been analyzed, meanwhile, the influence of tidal cycle on sediments and the overlying water, the effect of microalgae on nitrogen transfer across the sediment-water interface have been discussed in this work. The main findings are as following:(1)Based on the analysis of environmental factors, such as Eh (oxidation-reduction potential), Fe3+/Fe2+, and OC, this present work revealed the oxidation-reduction environment of the surface sediments in the Yangtze estuarine tidal flat, and showed that if taking no account of OC, the surface sediments of tidal flat in transitional area and saline area were under oxidation or less oxidation conditions, and the surface sediments of the other area were under reduction or less reduction conditions. (2)The temporal and spatial distribution of inorganic nitrogen near the interface between sediment and overlying water showed that the values such as Eh of sediments, exchangeable inorganic nitrogen of sediments and inorganic nitrogen in overlying water affected the distribution of NH4+-N and NO3--N+NO2--N in pore water; the distribution of exchangeable NH4+-N in the sediments had relationship with Eh and total nitrogen(TN); TN and OM(organic matter) had the same change trend and significant linear relationship with clay contents , it also indicated that sewage discharge , surface runoff and plant growth influenced the distribution of TN. (3)Taking into account temperature effect and sediment tortuosity effect, using Fick's first law can more accurately estimate diffusive flux of inorganic nitrogen across the sediment-water interface than previous reports. The result showed that NH4+-Nmainly diffused from the sediment into overlying water in the Yangtze estuarine tidal flat, indicating sediments was one of potential sources of NH4+-N in the overlying water; at the same time, the diffusive flux of NO3"-N+NO2"-N was mainly from the overlying water to sediment, reflecting that sediment was the important sink of NO3"-N+NO2"-N in the overlying water. Based on the analysis of diffusive flux and the other factors , it demonstrated that concentration of NH4+-N in the interstitial water dominated the distribution of NH4+-N diffusive flux; the concentration gradient of NO3'-N+NCY-N between interstitial water and overlying water mainly controlled the distribution of NO3"-N+NO2"-N diffusive flux.(4)In the core sediments of the study area, NH4+-N was the main component in dissolved inorganic nitrogen and exchangeable nitrogen, with a complicated profile change, and the concentration level of NO3'-N+NCY-N of dissolved inorganic nitrogen and exchangeable nitrogen decreased with increasing depth. These profiles suggested decomposition of organic matter mainly occurred in suboxic and anoxic conditions; in some cores, dissolved NHU+-N had significant positive linear relationship with exchangeable NH4+-N; in all of subsamples TN and OM had good relation; Affected by grain size, the degree of organic matter decomposition and sediment redox, the profile of exchangeable NH4+-N had bad relationship with TN. The early diagenesis model of NH4+-N in typical interstitial water profiles showed that the geochemical process during early diagenesis was mainly controlled by organic matter decomposition and clay mineral adsorption, which followed first-order kinetics.(5) The exchangeable NH4+-N varies with dissolved salinity in surface sediments during emersion period, and increased obviously during flooding in CM. After NH4+-N attended to the maximal concentration...
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