Wetlands In The Yangtze Estuary Biogenic Elements Nitrogen Biogeochemical Processes

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 between 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. With population's growth and economic development, lots of contaminants transported into estuarine and coastal zones have the negative influence on tidal water quality and the structures and functions of coastal ecosystem. Especially, more and more concerns have been focused on coastal eutrophication induced by the excessive input of nutrients.Nitrogen is one of key nutrient elements that result in the eutrophication and evolvement of estuarine and coastal ecosystems. Recently, anthropogenic input of nitrogen has already exceeded its natural origin in the estuarine and coastal regions, and furthermore the high loading of nitrogen is mainly retained in the areas. The coastal and tidal flats as important sources and sinks of nitrogen play key roles in nitrogen cycling in estuarine systems. However, the biogeochemical processes of nitrogen and their mechanisms in the complex systems remains unclear. Therefore, we chose the Yangtze estuary as a typical study area, and further studied the key processes of nitrogen cycling. The main findings are as following: (1) The magnitudes of the benthic exchange fluxes were determined on the basis of concentration gradients of ammonium and nitrate at the near-bottom water and interstitial water interface in combination with calculations of a modified Fick's first law. Ammonium fluxes varied from -5.05 to 1.43μg N cm^-2 d^-1 and were greatly regulated by the production of ammonium in surface sediments, while nitrate fluxes ranged from -0.38 to 1.36 ug N cm^-2 d^-1 and maybe were controlled by the changes in nitrate concentration gradients at the sediment-water interface. It was found that ammonium was mainly released from sediments into water columns at most of stations whereas nitrate was mostly diffused from overlying waters to intertidal sediments. In total, c. 823.75 t ammonium-N yr^-1 was passed from intertidal sediments to water while about 521.90 t nitrate-N yr^-1 was removed from overlying waters to intertidal sediments. This suggests that intertidal sediments had the significant influence on modulating inorganic nitrogen in tidal waters. (2) The influences of exposure to atmosphere on ammonium cycle in the intertidal surface sediments were in situ studied with a geochemical approach...