The Temporal And Spatial Distribution And Adsorption Characteristics Of Soil Phosphorus In The Three Gorges Water-level Fluctuation Zone

As a special area between the reservoir waters and the land,the special hydrological process of the Three Gorges Water-level-fluctuating zone often leads to the change of soil physical and chemical properties,resulting in the source-sink transformation of phosphorus between the soil and the water body.During the submerging period,the water-level zone constantly carries out a dynamic equilibrium process of phosphorus release and adsorption between the soil and the overlying water body.As one of the limiting factors of water eutrophication,phosphorus is the main limiting factor of algae population and density whose increase of its content directly leads to a large number of algae growth,thus affecting the water quality of reservoirs.Therefore,it is of positive significance to study the forms and adsorption-release characteristics of soil phosphorus in the water-level fluctuation zone at different altitudes along the main stream of the Three Gorges,which can deeply understand the characteristics of phosphorus migration and transformation in the water-level fluctuation zone and so that we can control phosphorus pollution,and protect the water environment in the Three Gorges Reservoir area.In this paper,105 soil samples in Banan-Zigui water-level fluctuation zone,the main stream of the Three Gorges Reservoir area,were taken as the research object.The phosphorus forms were analyzed by SMT（The Standards,Measurements and Testing）phosphorus classification method,and the basic physical and chemical characteristics of soil in water-leveling zone were tested and analyzed.The adsorption kinetics and thermodynamics experiments of phosphorus were carried out to explore the spatial and temporal distribution characteristics,occurrence forms,adsorption and release characteristics of phosphorus in water-leveling zone.The specific conclusions are as follows:（1）The median particle size in the study area varies from 9.90 to 154.78um and the spatial distribution shows a decreasing trend of sediment particle size from upstream to downstream,which is mainly affected by hydrodynamic factors.With the decrease of elevation,the median particle size of soil increased,and silt increased.The p H value of the soil in the water-level fluctuation zone is between 7.58 and 8.04,with an average value of 7.87 and the whole soil is weakly alkaline.The mass fraction of organic matter OM content in the study area is 2.54%～5.93%,with an average of4.12%.With the decrease of altitude,the mass fraction of soil organic matter OM content in the water-level fluctuation zone decreases,which are closely related to the vegetation coverage rate in the sampling area.The soil moisture content in the water-fluctuation zone varies from 13.46%to 21.39%.With the decrease of altitude,the soil moisture content shows a upward trend,and its distribution characteristics are related to the duration of soil flooding.（2）The TP content in the study area ranged from 178.918 to 1362.01mg/kg（mean800.36mg/kg）,and there was a large spatial difference in TP content from upstream to downstream.Land use and human activities along the Yangtze River were important factors affecting the soil TP content,whose content in the water-level fluctuation zone decreases with the increase of elevation,which may be caused by frequent dry-wet alternation and precipitation erosion in the Three Gorges reservoir area.In terms of interannual changes,the average content of TP in the water-level fluctuation zone of the main stream in the Three Gorges reservoir area remained at a high level（547.5～1006.26 mg/kg）.The change trend of soil TP content in the fluctuating zone of the main stream of the Three Gorges in recent ten years was relatively stable,and the annual increase and decrease range is small.The contents of IP,Fe/Al-P,Ca-P and OP in the soil in the water-level fluctuation zone are:445.87～874.33mg/kg（mean 654.9185mg/kg）,29.73～60.42mg/kg（mean44.85mg/kg）,352.40～824.98mg/kg（mean 605.03mg/kg）,89.45～268.35mg/kg（all157.08mg/kg）,accounting for 72.63%～93.04%,3.73%～8.17%,57.79%～87.79%and11.22%～27.37%of TP content respectively.The soil TP in the water-level fluctuation zone is mainly IP,accounting for 81.31%of TP,among which Ca-P is the main influencing form.The soil BAP content in the water-level fluctuation zone of the Three Gorges is in the range of 83.40～218.20mg/kg（mean 139.10mg/kg）,accounting for 13.48%～24.24%of the TP content,which has a certain risk of phosphorus release.There is a very significant positive correlation between TP and IP and Ca-P in the soil of water-level fluctuation zone,indicating that the increase of TP and IP content mainly comes from Ca-P.There was a significant positive correlation between soil Ca-P and p H in water-level fluctuation zone indicating that p H would affect the forms of soil phosphorus in the water-level fluctuation zone to some extent.There is a significant negative correlation between Fe/Al-P and grain size,which indicates that the grain size of soil in the water-level fluctuation zone to some extent affected the accumulation of Fe/Al-P in soil.（3）The adsorption process of phosphorus solution by the soil in the water-level fluctuation zone showed a fast adsorption stage within 0～2h,and a slow adsorption stage in 2～24h,after which the slow adsorption reached a dynamic equilibrium,and the fitting degree was good.The maximum adsorption capacity Q_max of soil in the water-level zone is 435.85～648.47mg/kg,and the difference of Q_max content between different regions is small.With the decrease of elevation,the maximum adsorption capacity of water-level fluctuation zone soil for phosphorus gradually increases.The background adsorption capacity NAP and adsorption-desorption equilibrium concentration EPC in the water-level fluctuation zone all increase with the decrease of elevation at different elevations indicating that the risk of phosphorus release in water-level fluctuation zone soil increases with the decrease of elevation.The concentration of soil EPC in the Three Gorges water-level fluctuation zone is generally lower than that in the overlying water of the Three Gorges main stream,and it is preliminarily determined that the soil in the Three Gorges water-level fluctuation zone is the"sink"of phosphorus in the water-soil system in summer.Correlation analysis showed that there was a significant negative correlation between soil adsorption rate K₂ and soil clay content,indicating that the rapid adsorption of soil phosphorus in the water-level fluctuation zone was generally physical adsorption.The maximum phosphorus adsorption capacity Q_max is positively correlated with the silt content and negatively correlated with the sand content.With the decrease of elevation,the proportion of clay and silt in the water-level fluctuation zone gradually increases,while the OM content gradually decreases,which makes the phosphorus adsorption capacity of the water-level fluctuation zone soil increase with the decrease of elevation.