Characteristic Curve Model Of Salinized Soil Based On Pore Structure

Soil salinization is a global ecological environment problem,especially in arid and semi-arid areas.The change of macropore structure of salinized soil leads to soil degradation,affects crop nutrient absorption,leads to crop yield reduction,and affects agricultural productivity.However,there are few studies on soil water characteristic curves and macropore characteristics under different salinization degrees.In this paper,the soil in three experimental areas with different salinity gradients(no,mild and severe salinized soil,expressed by YZ1,YZ2 and YZ3)in shahao canal irrigation area of Hetao irrigation area in inner Mongolia was taken as the research object,and the water characteristic curves of undisturbed saline soil with different salinization degrees were measured and analyzed,and the differences of hydraulic characteristic parameters of undisturbed soil with different salinization degrees were revealed.The pore structure parameters were extracted by non-destructive X-ray tomography(CT)and Avizo 9.0 software,and the visual macropore network was reconstructed.The differences of pore structure parameters such as macropore and macropore number in undisturbed saline soil of various representative plots under different pore diameters were clarified.By using the method of Partial Least Squares(PLS)Variable Importance Projection Value(VIP),the important factors affecting the number of macropores and macropores were screened,and the relationship between pore structure parameters and salt content of undisturbed saline soil with different salt content was explored.Finally,combining fractal model with CT scanning method,a fractal model for predicting soil moisture characteristic curve at pore scale was established.The main research results are as follows:(1)On the whole,VG model fits well the soil-water curve of 0-60cm soil layer in YZ1,YZ2 and YZ3 plots,but the fitting accuracy of YZ2 plot is less than that of YZ1 and YZ3 plots.The equivalent pore size distribution in 20cm of YZ2 soil is similar to that in YZ1soil,but the micro-pores and the proportion of micro-pores in total pores in 40cm and 60cm depth are increased compared with that in YZ1 soil.However,the ratio of microporosity and microporosity to total porosity in 0-60cm depth of YZ3 soil is smaller than that in YZ1 soil.The field water holding rate of 40 cm and 60cm soil layers in YZ2 soil is higher than that in YZ1 soil layer.With the increase of soil depth,the field water holding capacity of YZ3 soil is less than that of YZ1 soil.(2)The quantity,volume,area and porosity of undisturbed soil with different salinization degrees were quantitatively studied.With the increase of soil depth,the number of macropores in YZ1 and YZ2 decreased,while the number of macropores in YZ3increased first and then decreased.With the increase of soil salinization degree,the soil porosity decreases,ranging from 0.523%to 3.682%.On the whole,with the increase of soil depth,the soil porosity also decreases.Based on partial least squares regression PLS and variable re-projection VIP method,regression and importance screening are carried out for the large porosity,the number of large pores and influencing factors.The R~2 values of the regression models of large porosity and large porosity are 0.82 and 0.71,respectively,with high accuracy.The important influencing factors of large porosity are conductivity,penetration resistance and powder content,in which the VIP values of conductivity and penetration resistance are 1.37 and 1.20;The important factors affecting the number of macropores are electrical conductivity,powder content and sand content,and the VIP value of electrical conductivity is 1.51.Macroporosity and the number of macropores are only significantly correlated with conductivity(P<0.01),and their correlation coefficients are-0.808 and-0.852 respectively.(3)Establishing pore scale KZ-BC fractal model to predict the water characteristic curve of undisturbed saline soil.The fractal dimension of soil three-dimensional pores under different salinization degrees and depths ranges from 2.547 to 2.912;With the increase of soil salinization,the fractal dimension of soil three-dimensional pores decreases.The fractal model R~2 of KZ-BC for predicting the water characteristic curve of undisturbed saline soil is larger(0.825-0.996).RMSE of root mean square error is 0.006-0.111,and MAE of average absolute error is 0.0016-0.1087,which indicates that the model prediction is more accurate.