Study On Transport Parameters Of Soil Nutrients Driven By Water In Black Soil Region

Environmental pollution caused by solute migration is a hot topic in agricultural soil and water engineering.Northeast black soil area is an important grain production area in our country.The use of chemical fertilizer and pesticide has significant influence on grain production and groundwater pollution.Soil pollution caused by excessive application of chemical fertilizer and soil secondary salinization are the key problems that need to be studied and solved in this field.On the basis of referring to domestic and foreign literatures,this paper determined the soil nutrient transport parameters through unidirectional soil nutrient diffusion experiment in water,surface soil nutrient diffusion experiment in water and indoor soil column experiment,and combined with the application of mechanism model to verify the accuracy of the parameters.Through the research,the thesis obtains the following main conclusions.（1）Unidirectional diffusion experiment of soil nutrients in water,the maximum diffusion rate and minimum diffusion rate of Br-are 0.91cm/min and 0.47cm/min when the water temperature is20℃and the concentration and diffusion distance are different.The maximum diffusion rate of NO₃^-is 0.33cm/min.The minimum diffusion rate is 0.23cm/min.The diffusion velocity of Br^-is higher than that of NO₃^-in water with different concentration of solute.Under the same concentration condition,the longer the diffusion distance,the longer the ion diffusion time.The minimum and maximum diffusion rates of Ca²⁺are 0.42cm/min and 0.75cm/min.The minimum diffusion rate of K⁺is 0.49cm/min,and the maximum diffusion rate is 1.02cm/min.In the unidirectional diffusion test,the diffusion velocity of K⁺in water is greater than that of Ca²⁺when the concentration and diffusion distance change.The linear relationship between the diffusion velocity and concentration of NO₃^-and K⁺in water is established.It is assumed that the hydrodynamic dispersion coefficients of different ions are proportional to each other and equal to the ratio of the diffusion velocity at the same concentration.It is assumed that the hydrodynamic dispersion coefficients of NO₃^-and K⁺are 0.629×10^-5～3.45×10^-5cm²/h and 2.4×10^-5～2.72×10^-5cm²/h.（2）Soil nutrients in the water in planar diffusion experiment,the maximum diffusion rate of Br-was 65.74cm/min in the surface diffusion test of soil nutrients in water.The minimum diffusion rate is 29.42cm/min,and the maximum diffusion rate of NO₃^-is 67.62cm/min.The minimum diffusion velocity is 22.19cm/min.The maximum diffusion rate of K⁺is 56.8cm/min and the minimum diffusion rate is 7.47cm/min.The maximum diffusion rate of Ca²⁺is 65.13cm/min,and the minimum diffusion rate is 7.22cm/min.The diffusion of K⁺and Ca²⁺varies with the distance,showing different characteristics.With the increase of the diffusion distance,the increase of the diffusion time gradually becomes larger.The longer the diffusion distance,the longer the diffusion time of the two solutes.In the planar diffusion test in water,the ef fect of diffusion direction on the diffusion of the same solute is not significant.The linear relationship between diffusion velocity and concentration of NO₃^-and K⁺in water is established.It is assumed that the hydrodynamic dispersion coefficients of different ions are proportional to each other and equal to the ratio of the diffusion velocity at the same concentration.The hydrodynamic dispersion coefficients of NO₃^-and K⁺are assumed to be 1.24×10^-5～1.344×10^-5cm²/h and 1.068×10^-5～1.371×10^-5 cm²/h.（3）In the indoor soil column experiment,the greater the concentration of the replacement liquid,the more obvious the left deviation of the penetration curve,the sho rter the initial penetration time and the shorter the complete penetration time.The higher the concentration is,the more the transmission curves of NO₃^-and Br^-move to the left,and the easier it is for NO₃^-and Br^-to penetrate the soil,because both NO₃^-and Br^-are non-reactive ions in the soil.Under the action of different concentrations of Ca Cl₂ solution,the larger the inflow concentration,the smaller the average pore velocity of Ca²⁺,but the dispersion degree increases accordingly,and the change of blocking factor R is not very significant.Because of homogeneous loam soil is uniform and the relatively small pore between soil particles,the breakthrough curve of Ca²⁺is also obvious at the same time,the phenomenon of the trailing,through to the degree of dispersion and average flow velocity of pore,quality exchange coefficient,two flow region,the sensitivity analysis of the relative size,velocity ratio,it is concluded that the greater the degree of dispersion,solute the smaller the r elative concentration of bimodal;With the decrease of the average pore velocity,the double peak of the penetration curve becomes less and less obvious.When the velocity ratio is equal to 1,the two-flow region model evolves into the traditional one-region model.（4）Based on the above unidirectional and planar diffusion tests of Br-,NO₃^-and K⁺in water,the hydrodynamic dispersion coefficients of NO₃^-and K⁺are 0.629×10^-5～3.45×10^-5cm²/h and1.068×10^-5～2.72×10^-5cm²/h.The hydrodynamic dispersion coefficient of NO₃^-is 2.0395×10^-5cm²/h,and that of K⁺is 1.894×10^-5cm²/h.Input into model CXTFIT2.1 to simulate the NO₃^-and K⁺penetration process of indoor soil column,the square of correlation coefficient is 0.98 and 0.97,RMSE is 0.235×10^-5cm²/h and 0.245×10^-5cm²/h.The availability of NO₃^-and K⁺hydrodynamic dispersion coefficients is verified.Meanwhile,the availability of other parameters of CXTFIT2.1 is verified by using the Twolong model.The research results and theoretical analysis in this paper are helpful to improve people’s understanding of the process and mechanism of soil nutrient migration in black soil area,and lay a scientific foundation for rational irrigation and fertilization,increase of yield an d control of groundwater pollution in black soil area.