Effects Of Phosphorus Input On Organic Carbon In Sediments Of The Yangtze River Estuary

The pollutants discharged by human activities will enter the ocean through underground runoff,river transportation and other means,and the phosphorus will be adsorbed in the sediments,so as to accelerate the mineralization of organic carbon in the sediments,which is not conducive to the fixation of carbon.Although many scholars have found that phosphorus deposition can promote the degradation of organic carbon,the research on its internal mechanism is still insufficient.In this study,different amounts of sodium superphosphate were added as exogenous phosphorus to simulate the effect of phosphorus on the production of organic carbon in the sediments of the near shore and sea area of the Yangtze River Estuary.Under the condition of different amount of exogenous phosphorus input,the exchange characteristics of phosphorus at the sediment water interface and the decomposition and dissolution of organic carbon in sediments with the input of exogenous phosphorus are studied.Finally,the effect of Exogenous Phosphorus Input on the structure of dissolved organic matter in sediments is studied by UV-Vis absorption spectrum,which is described from the changes of humification degree,aromatization degree,molecular weight and aromatic ring structure of organic matter.The similarities and differences of sediment changes under Exogenous Phosphorus Input in two different regions of the Yangtze River estuary are compared.The main conclusions of this paper are as follows:(1)When exogenous phosphorus enters the water body,it will break the phosphorus balance between solid and liquid.When the phosphorus content in the overlying water is high,the phosphorus in the water body will continue to accumulate in the sediment,resulting in the decrease of phosphorus concentration in the overlying water and the increase of phosphorus content in the sediment.The fitting model between phosphorus flux and phosphorus adsorption at the sediment water interface shows that the accumulation rate of overlying water with different phosphorus concentrations in the sediment is different after the exogenous phosphorus enters the water body.Due to different sediment properties and solutes in the overlying water,the adsorption of phosphorus in the overlying water by the sediments of the two places is also different.The adsorption capacity of estuarine sediments to phosphorus in the overlying water is greater than that of marine sediments.It takes longer for marine sediments to reach the adsorption equilibrium,and the phosphorus content in the overlying water is higher at the equilibrium.(2)Through the analysis of carbon and phosphorus content in the sediments of the Yangtze River Estuary through field investigation,it is found that due to the interaction between inorganic phosphorus and total organic carbon in the sediments,the increase of organic matter content covering the sediment surface can reduce the fixation of inorganic phosphorus,and the increase of inorganic phosphorus content will promote the mineralization of organic matter.Therefore,there is a significant negative correlation between them.The decreasing rate of total organic carbon content in the sediments of the Yangtze River Estuary will increase with the increase of exogenous phosphorus input.On the 56 th day after the Exogenous Phosphorus Input experiment,the TOC content of sediments in each test group decreased by 27.5%,33.5%,34.3%,34.8%,36.9% and 39.4% respectively;The TOC content of sediments in each experimental group of the estuary decreased by9.89%,13.37%,14.38%,15.62% and 18.90% respectively.In contrast,due to the difference of organic matter in sediments,the mineralization rate of organic carbon in marine sediments is higher.At the beginning of the experiment,TOC in the overlying water of the estuary group was positively correlated with the amount of phosphorus input,but with the extension of time,this correlation gradually weakened,while there was no such phenomenon in the sea area group.Therefore,it can not explain the relationship between the amount of exogenous phosphorus input and TOC content in the overlying water.(3)The input of exogenous phosphorus can promote the effective degradation of organic matter components in marine sediments,reduce the degree of sediment humification in the Yangtze River Estuary and improve the bioavailability.However,from the perspective of carbon sink,this phenomenon is not beneficial to the fixation of Sediment Carbon Pool in the Yangtze River Estuary.The decomposition of organic matter components is essentially the change of dissolved organic matter structure.The humification degree,aromatization degree,molecular weight and aromatic ring structure of sediment dissolved organic matter were characterized by four UV spectral indexes such as E280,E254,E250 / 365 and E253 / 203.Under the action of phosphorus,the degree of humification and aromatization in the coastal sediments of the Yangtze River Estuary first increased and then decreased;The degree of humification and aromatization of marine sediments are decreasing.The molecular weight of dissolved organic matter in the sediments of the two places is decreasing,and there is a phenomenon of transformation from macromolecules to small molecules;The activity of aromatic ring structure is increasing.At the same time,with the increase of exogenous phosphorus input,the change speed and range also increase.It shows that the increase of exogenous phosphorus input can accelerate the change of sediment dissolved organic matter.At the same time,the high linear fit in the linear relationship between the UV spectral index of dissolved organic matter structure and the total phosphorus content of sediments supports the influence trend of the above Exogenous Phosphorus Input on the mineralization of total organic carbon in the sediments of the Yangtze River Estuary.Therefore,the input of exogenous phosphorus can promote the effective degradation of organic matter components in marine sediments,reduce the degree of sediment humification in the Yangtze River Estuary and improve the bioavailability.However,from the perspective of carbon sink,this phenomenon is not beneficial to the fixation of Sediment Carbon Pool in the Yangtze River Estuary.Therefore,further controlling the input of exogenous phosphorus can increase the carbon sequestration of sediments in the Yangtze River Estuary.