Experimental Study On Phosphorus Release Characteristics Of Four Alluvial Soils With Different Texture

During the rainfall,the surface soil of farmland is fully mixed with runoff under the disturbance of raindrop splash and runoff erosion,and the nutrients in soil are easily lost with runoff,resulting in agricultural non-point source pollution.The dissolved phosphorus entering the runoff through desorption has the most direct impact on the quality of water environment.The release of phosphorus from soil to water is a complex process affected by many factors,which is closely related to the rainfall-runoff condition,soil basic properties and adsorption characteristics.It is helpful to understand the mechanism of soil phosphorus loss by simulating the release of phosphorus to water under the condition of surface runoff.Therefore,in this study,soils with different texture in Danjiangkou Reservoir Area was taken as the research object.Through the comparative study of phosphorus release characteristics under different water/soil ratio,shaking time and initial phosphorus content,the change law of soil phosphorus release under different surface runoff conditions was explored.Combined with the basic physical and chemical properties of soil and soil phosphorus adsorption characteristics,the influencing factors of phosphorus release were explored.The main conclusions are as follows:（1）Under the condition of 25:1 water/soil ratio and 2 h shaking time,the release of phosphorus from 20 soil samples in Danjiangkou Reservoir Area was affected by clay,silt,sand,p H,total phosphorus and available phosphorus,which was negatively correlated with clay,silt and p H,and positively correlated with sand,total phosphorus and available phosphorus.The release of phosphorus can be predicted by sand content and available phosphorus content.The prediction equation is Pd=-2.736+0.890*Olsen-P+0.150*Sand,and the prediction accuracy is 93.2%.（2）The adsorption curves of four different soil samples showed the same change law,and the adsorption amount of phosphorus increased with the increase of equilibrium concentration.The results show that Langmuir equation is the best fit method.Among the four soil samples,the adsorption saturation DPS of sandy loam was the highest,indicating that the risk of phosphorus loss was the highest,followed by loam,clay loam,and silty clay.For EPC₀（zero suction balance concentration）,the order is sandy loam>loam>clay>silty clay,ranging from 0.047 to 2.318 mg/L,which is far beyond the critical value of 0.02 mg/L that leads to the eutrophication,indicating the potential threat to water environment.（3）Under any water/soil ratio and shaking time,the phosphorus release of four soil samples showed the same rule:sandy loam>loam>clay>silty clay.The change trend of phosphorus release in four soils was consistent with time,which showed the trend of rapid release first,then slowly release,and the release increased with time.In the first60 minutes,the release amount increased greatly,and then 60 min-240 min,it increased slowly.The logarithm of phosphorus release amount and the logarithm of shaking time were significantly linear correlation under any water/soil ratio.The phosphorus release amount of four soil samples increased with the increase of water/soil ratio.When the water/soil ratio was between 10:1 and 200:1,the release amount increased rapidly with the water/soil ratio,and it increased slowly when the water/soil ratio was between 200:1and 500:1.The logarithm of release amount and the logarithm of water/soil ratio had a significant linear correlation.（4）The release of phosphorus increased with the increase of initial phosphorus content.The results showed that the relationship between Olsen-P and release amount was significant in the water/soil ratio of 25:1 and different time.Based on the above correlation,the initial phosphorus content,shaking time and water/soil ratio can be used to predict the phosphorus release amount.And the prediction equations of four soils are obtained as follows:P_d=0.533P₀t^0.068W^0.437 for sandy loam,P_d=0.465P₀t^0.099W^0.584for loam,P_d=0.285P₀t^0.181W^0.638 for clay,and P_d=0.280P₀t^0.179W^0.614 for silty clay,which can be used for prediction of phosphorus release amount in the time range of 5-180 min and water/soil ratio range of 10:1-500:1.