Nitrogen Forms And Their Functions In Biogeochemical Cycling In Bohai Sea Sediments

Based on the sequential extract process, nitrogen forms for natural grain in Bohai Sea sediments and their functions in nitrogen biogeochemical cycling are studied in this thesis. Four forms of nitrogen (ion-exchangeable form (LEF) , carbonate form (CF) ,iron-nunganese oxides form (LMOF) and organic matter-sulfide form (OSF)) are determined, and their contributions to nitrogen cycling in marine system are evaluated. LEF-N, CF-N, IMOF-N and OSF-N account for 3.67%, 0.31%, 0.43% and 26.45% of total nitrogen(TN) in the sediments, respectively, and their distributions are controlled by different factors. Distribution of NH4 in ]EF-N is mainly affected by OC, Es and clay property, N03 is controlled by water distribution. Provenance, clay property and sediment size are the main factors influencing OSF-N distribution. Some heavy metals such as Fe, Mn, Co, Cu also show influence on nitrogen forms distribution. Considering some control parameters such as sediment accumulation rate, QC, Es, redox environment, nitrogen decomposition and burial process in surface and column sediments are characterized as follows: (1) there is a strong correlation between nitrogen burial flux and sediment accumulation rate, about 69.15% of nitrogen become relatively stable state after early diagensis. (2) a large amount of inorganic nitrogen combined within the sediment is the main reason that causes low C/N ratio. And the low N/P ratio is due to increased terrestrial discharge of P and higher burial efficiency of P to N. (3) calculations show that decomposition rate constants of C, N, P and Si in Bohai Sea sediments have the sequence N>P>C>Si. Estimation of transferable nitrogen and their functions in sediment-water exchange shows that contributions of different nitrogen forms in nitrogen recycling vary with time scale. The absolute contribution of each state is consistent with its content in sediment, which has the sequence OSF-N>IEF-N>IMOF-N>CF-N. Estimation indicates that LEF-N and OSF-N exist mostly in surface of sediment grain, and act as dominant participator in recycling, IMOF-N and CF-N mainly present inside of grain and contribute little to recycling. Nitrogen cycling budget for Bohai Sea shows that contribution of pelagic recycling (74.4%) to primary productivity is much more than benthic recycling (26.1%) Correlations between sediment nitrogen and prixnary production show that there is a tight link between these two aspects. IEF-N acts as a uffer pooP for nitrogen nutrient which can improve primary productivity. Benthos bioturbation conduce to enhancement of mineralization, which results in high NH4 and N03 distribution in deep-seated sediments.