Study On Nitrogen Fraction And Release Potential Of Surface Sediments In The Bohai Sea

As an important"source"and"sink"of nutrients such as nitrogen in marine ecosystem,sediments play an important role in balancing nutrient concentration and nitrogen cycle in overlying water.Nitrogen is an important element in the nutrient cycle of marine ecosystem.The change of its content and proportion will affect the community structure and growth of phytoplankton and algae in water,and become a restrictive factor for the growth of phytoplankton.According to the bulletin on China’s ecological environment in 2019,dissolved inorganic nitrogen（DIN）has become one of the main substances exceeding the standard in the Bohai Sea,while the inorganic nitrogen in the Yellow Sea and Bohai Sea mainly comes from sediments.The existing forms of nitrogen in sediments play different roles in the nitrogen cycle,and the amount of nitrogen that can be released is also different.Ammonia nitrogen and nitrate nitrogen are the main forms of nitrogen exchange at the sediment water interface.Studying the release process of ammonia nitrogen from sediments and the fitting of release flux is of great significance to understand the nitrogen cycle in sediments and to evaluate environmental quality.Based on the surface sediments collected in April 2019 in the Central Bohai Sea,total nitrogen（TN）was divided into ion-exchange nitrogen（IEF-N）,weak acid leachable nitrogen（WAEF-N）,strong alkali leachable nitrogen（SAEF-N）and strong oxidant leachable nitrogen（SOEF-N）by continuous fractional leaching and separation method.The contents of different convertible nitrogen in surface sediments were measured and their spatial distribution differences were analyzed,The influence mechanism of various forms of nitrogen and its ecological effect on marine overlying water were discussed.At the same time,the effects of temperature,oscillation frequency and pH on the release of DIN in surface sediments were studied based on response surface method.The main results and conclusions are as follows:（1）The spatial distribution of nitrogen speciation varied greatly,and in general,the content of convertable nitrogen（TTN）ranged from 76.22 to 129.49 mg/kg,and the content of TTN was SOEF-N>IEF-N>SAEF-N>Wa EF-N,the distribution differences of nitrogen forms are mainly affected by the input differences of land-based organic matter and the flow direction of ocean currents,while TN is mainly affected by the regional organic matter input content.Regions with higher TTN levels indicate that this region has stronger exogenous inputs,and regions with higher IEF-N levels have stronger nitrogen releases.Correlation analysis showed that the content of SOEF-N,TTN and NTN was mainly controlled by the content of TN and powder particles,while the correlation with the content of Ca and Mn was small,and the content of metal elements Al and Fe had a greater impact on the content of IEF-N.（2）The influencing factors and release potential of transformable nitrogen（TTN）.The nitrogen in the surface sediments of the Bohai Sea was divided into ion-exchange nitrogen（IEF-N）,weak acid leachable nitrogen（WAEF-N）,strong alkali leachable nitrogen（SAEF-N）,strong oxidant leachable nitrogen（SOEF-N）and non transformable nitrogen（NTN）.The effects of sediment particle size and organic matter on the amount of nitrogen released were studied,The effects of TTN release from sediments on dissolved inorganic nitrogen（DIN）and chlorophyll a（Chla）in overlying water were discussed.The results showed that the content order of TTN was SOEF-N>SAEF-N>IEF-N>WAEF-N,the release amount was IEF-N>SAEF-N>SOEF-N>WAEF-N,and the release ratio was IEF-N>WAEF-N>SAEF-N>SOEF-N.69.83%of IEF-N,64.93%of WAEF-N,56.27%of SAEF-N and 29.56%of SOEF-N will be released again.Pearson correlation results showed that the release of SOEF-N in sediments was negatively correlated with silt content,but positively correlated with sand content;SOEF-N is negatively correlated with Chla and dissolved inorganic nitrogen（DIN）in overlying water,indicating that the release of SOEF-N has little effect on it,while Chla and DIN in surface and bottom water have strong positive correlation respectively（r=0.803,P<0.01;r=0.831,P<0.01）,which may mean that there is no obvious stratification of water column in the study area.（3）In order to study the effect of environmental factors on the exchange of dissolved inorganic nitrogen（DIN）at the sediment overlying water interface in the Bohai Sea,the effects of temperature（T）,oscillation frequency(r·min^-1)and pH on the exchange flux of DIN were studied by single factor experiment and response surface experiment.The results of single factor experiment show that the din flux increases with the increase of temperature and reaches the maximum at 30℃;The oscillation frequency of NO₃^-flux increases with the increase of NO₃^-+NO₂^-,and then decreases with the increase of NO₃^-;NH₄⁺flux decreases with the increase of pH,increases after reaching the minimum at pH=7,and NO₃^-increases with the increase of pH.The fitting results of Box Behnken（BBD）model show that the fitting relationship between environmental factors and DIN flux is quadratic polynomial,and the model is significant,which has practical significance.The results of response surface experiment showed that temperature had a significant effect on NO₃^-+NO₂^-flux（P<0.05）;The interaction of temperature and oscillation frequency had a significant effect on NH₄⁺flux（P<0.05）.T=6.33℃,r=156.16,r·min^-1,pH=7.14 are the most unfavorable conditions for sediment flux exchange in the Bohai Sea.The diffusion fluxes of NO₃^-+NO₂^-and NH₄⁺are2.42 mg·（m²·h）^-1 and 2.99 mg·（m²·h）^-1 respectively,and the minimum response values are obtained.In summary,the bohai sea surface sediments are the nitrogen source of the Bohai seawater,that is,the amount of sediment that DIN sinks from the water is less than the amount released from the sediment into the water.