Study On Phosphorus Release From Soil Of Water-level-fluctuation Zone And Sediment In Dongzhang Reservoir

The water-level-fluctuation zone and sediment,as the transition zone and reservoir of non-point source pollution,are the"source"and"sink"of nitrogen,phosphorus and other nutrients.In the process of periodic fluctuation of reservoir water level,the nutrient salts in soil and sediment will exchange with the overlying water through interstitial water,and then affect the water quality and water ecology of the overlying water.Based on the analysis of water quality and phytoplankton structure characteristics in Dongzhang Reservoir from 2020 to 2021,this study further analyzed the distribution characteristics of various phosphorus forms in the soil of water-level-fluctuation zone and sediment of Dongzhang Reservoir,and simulated the phosphorus release capacity of the water-level fluctuation zone soil and sediment samples under the influence of different environmental factors by laboratory culture method.Finally,the annual release of phosphorus in the fluctuation zone and sediments of Dongzhang Reservoir and its contribution to the overlying water are estimated.The results are as follows:The mean total phytoplankton density(2.3×10~7～5.2×10~7 cells/L)at the dam site during the wet season in 2021 was more than three times that of the mean phytoplankton density(5.6×10~6～2.3×10~7 cells/L)during the dry season.Bacillariophyta,Cyanophyta and Chlorophyta dominated in dry season,with an average proportion of 47.5%,31.6%and 14.0%respectively,while Cyanophyta dominated in wet season,with an average proportion of 77.7%.By analyzing the content of various forms of phosphorus in water samples in different hydrological periods,it is found that the content of soluble reactive phosphorus(SRP)is low in wet season and dry season,but the contents of total phosphorus(TP),total dissolved phosphorus(TDP)and dissolved organic phosphorus(DOP)are high.The results of correlation analysis and redundancy analysis show that there is a negative correlation between total algae,Cyanophyta,Chlorophyta,Pyrrophta and SRP,indicating the utilization of phosphorus nutrients by algae in Dongzhang Reservoir.According to the analysis results of SMT extraction method,the contents of total phosphorus in the soil of water-level-fluctuation zone and sediments in Dongzhang Reservoir are 46.3～357.9 mg/kg and 184.5～1348.8 mg/kg respectively.The average content of total phosphorus in sediments is 2.83 times that of the water-level-fluctuation zone.Among them,the organic phosphorus in water-level fluctuation zone soil and sediment accounts for 55.3%and 26.6%of the total phosphorus respectively,while the iron aluminum bound phosphorus,as an important part of the inorganic phosphorus in the water-level-fluctuation zone and sediment,accounts for 88.9%and 83.9%of the inorganic phosphorus respectively,indicating that the water-level-fluctuation zone and sediment of Dongzhang Reservoir have great release potential.Under the influence of environmental factors,SRP and TDP in the water-level fluctuation zone and sediments were the source of phosphate in overlying water at 15℃and 35℃.The higher the temperature,the greater the exchange flux.At 25℃,the overlying water concentration was 0,0.02 and 0.06 mg/L,respectively.The results showed that when the overlying water content was low,the phosphorus in water-level fluctuation zone soil was the"source"of overlying water phosphate.With the increase of overlying water concentration,the effect of soil on overlying water phosphate changed from"source"to"sink".Under the condition of 30℃,the release of SRP and TDP in sediments increased with the addition of organic phosphorus solubilizing bacteria.The release capacity of organic phosphorus solubilizing bacteria to SRP and TDP in sediments showed a general trend of Bacillus>Pseudomonas>original microbial action in sediments.Finally,the contribution rate of total dissolved phosphorus in the water-level fluctuation zone of Dongzhang Reservoir to total phosphorus in overlying water is11.2%～17.4%,and that of total dissolved phosphorus in sediments to total phosphorus in overlying water is 14.9%～31.1%.In addition,the annual phosphorus release in the fluctuation zone and sediment of Dongzhang Reservoir is 0.0019 t/a and 0.0384 t/a,respectively.