Characteristics Of Sodic Alkaline Soil Water Infiltration Process And Water-salt Transport

Infiltration is the process by which irrigation water or rainfall enters the soil from the surface.The infiltration process is not only the redistribution process of water in the soil,but also the migration and transformation of pesticides,pollutants,nutrients and other substances.For saline soil,the dynamic change of salinity caused by the change of moisture during the infiltration process is the core issue in the study of saline soil improvement.The Songnen Plain is one of the three major saline soils in the world.The problem of salinization is an important factor restricting regional economic development and improving the ecological environment.Studying the infiltration process of saline soil with different saline-alkali levels,and discovering its influencing factors and mechanisms,so as to improve and utilize it,is of great significance to promote regional salinization prevention and rational use of water and soil resources.This study takes soda-alkali soils with different salinity levels represented by five typical saline landscapes in the western Songnen Plain as the research object,combined with field monitoring and indoor simulation,using single-ring constant head water infiltration method and dye tracer method to detect the infiltration process of five typical saline landscape soils was analyzed,and the comprehensive index describing the infiltration process was determined through principal component analysis,and the infiltration capacity index was constructed;and the environmental factors affecting the infiltration process were determined through the structural equation model.The one-dimensional soil column vertical infiltration test system analyzes the influence of biochar addition amount and particle size on the infiltration process.Finally,by verifying the adaptability of the four classic infiltration models to simulate the test soil infiltration law,the best model with a high degree of fit was selected.This article mainly draws the following conclusions:(1)The fractal dimension can be used as an indicator to reflect the texture and salt content of the tested soil.The soil salt content of the six saline landscapes is between 569.67?14311.78 mg·kg^-1(0.06%?1.43%).The content of HCO₃^-and Na⁺in the salt composition account for 57.24%and 14.68%,respectively,and the p H is between 8.68?10.35.EC_e<4 d S·m^-1,the SAR of the other five different saline landscape soils is between 21.92 and 115.71,all of which are typical soda-alkaline soils.The sand content is the highest in the texture composition,and the fine sand content is mainly 49.48%?77.78%.The fractal dimension is between 2.35 and 2.61.The fractal dimension has a very significant negative correlation with the sand content,and is related to powder and clay.The content showed a significant positive correlation.The fractal dimension is significantly positively correlated with the salt content and extremely significantly positively correlated with the HCO₃^-content.The salt and HCO₃^-content are significantly correlated with the soil particle composition.(2)The steady soil infiltration rate of the five saline landscapes decreases with the increase of soil alkalinization.The alkalinity of corn field is the smallest and the stable infiltration rate is the largest(65.10?4.29 mm/h),followed by Aneurolepidium chinese grassland soi(47.92?6.44 mm/h),there is a significant difference between the two(P<0.05).The other three types of soil stable infiltration rate are:the soil of Chloris virgata grassland soil(1.78?0.44 mm/h)>Bare land of saline-sodic soil(0.28?0.33 mm/h)>Suaeda glauca soil(0.13?0.10 mm/h),there are no significant differences among the three stable infiltration rate.The difference is that when stable infiltration is achieved,the infiltration rate of the five soils is only 60.7%,36.1%,2.2%,1.0%and 0.8%of the initial infiltration rate,respectively.The average infiltration rate and the stable infiltration time are used to establish the infiltration capacity index that comprehensively characterizes the infiltration process.The structural equation is established based on the four parameters of fractal dimension,non-capillary porosity,sodium adsorption ratio and salt content.The model explains97%of the infiltration capacity index,in which salt content and fractal dimension have a negative effect on infiltration,while non-capillary porosity promotes infiltration.The negative effect of salt(-0.92)is much greater than thr negative effect of the fractal dimension(-0.16)and the promotion effect of non-capillary pores(0.18).Comparing the four classic infiltration models,the Horton model has a high degree of fitting to the soil water infiltration process of corn field,Aneurolepidium chinese grassland,Chloris virgata grassland and Bare land of saline-sodic soil with R²ranged from 0.93 to 0.97,and the RMSE is the smallest,ranged from 3.34 to 7.20.The general empirical model has a high degree of fitting to the soil water infiltration process of Suaeda glauca,R²and RMSE are 0.99 and 0.34,respectively.The predicted value of the model and the measured value by the Chi-square test show that there is no significant difference between the two,so the Horton model and the general empirical model can be used to simulate the infiltration law of the tested soil.(3)The five different degrees of soda-alkaline soil infiltration water flow patterns are mainly matrix flow,and priority flow also has different degrees of development.The nature of the soil itself is the main reason that affects the soil water flow movement pattern.The typical parameters of soil salinization(salt content,HCO₃^-content,alkalinity degree,sodium adsorption ratio)and preferential flow parameters are significantly or extremely significantly negatively correlated;soil texture(fractal dimension)and preferential flow parameters are also extremely negative.Significant negative correlation;the coarser the soil texture,the less prone to preferential flow.Soil bulk density and total porosity are significantly negatively correlated and extremely significantly positively correlated with preferential flow parameters,respectively,but there is no significant correlation between soil initial water content and preferential flow parameters.The water content of the 0-10 cm soil layer after infiltration was significantly higher than that before infiltration,with an average height of 0.98%?21.17%.As the depth of the soil layer increased,the water content after infiltration gradually approached the soil moisture content before infiltration.The electrical conductivity of the 0-10 cm soil layer after infiltration is3.39%?63.30%lower than that before infiltration,and the electrical conductivity at the maximum infiltration depth is increased by 11.68%?85.89%,except for farmland sandy soil which decreases by 20.72%.The salt in the surface soil is leached into the deep soil to varying degrees during the infiltration process.(4)The effect of biochar on the infiltration process is not only related to the amount and particle size of the addition,but also related to the texture of the soil itself.When the particle size is less than 0.25 mm and the addition amount is 5%,the addition of biochar reduces the stable infiltration rate of corn field and Aneurolepidium chinese grassland,while the addition amount of 10%and 15%increases the stable infiltration rate.While the particle size is 0.25-1mm and 1-2mm,adding 5%,10%,15%all reduce the stable infiltration rate of corn field and sheep grassland soil.The addition of biochar consistently promoted the stable infiltration rate of Chloris virgata grassland and Bare land of saline-sodic soil.The increase range of stable infiltration rate for Bare land of saline-sodic soil and Chloris virgata grassland was 11000.00%?5650.00%and 200.00%?342.86%.For Bare land of saline-sodic soil,the stable infiltration rate increased in the range of11000.00%?5650.00%and 200.00%?342.86%.The promotion effect of the stable infiltration rate of sandy loam soil is far greater than of the sandy soil of loam soil,and the promotion effect of small particle size biochar on the infiltration of Chloris virgata grassland and Bare land of saline-sodic soil is significantly higher than that of large particle size biochar.Compared with no biochar,the addition of biochar increased the electrical conductivity of the same soil layer in the corn field and Aneurolepidium chinese grassland,and the increase increased with the increase of the amount of biochar.The addition of biochar reduced the electrical conductivity value of the Chloris virgata grassland soil and Bare land of saline-sodic soil,and the greater the amount added to the same soil depth,the greater the reduction.