Assessment On The Uniformity Of Purple Soil Profile On Slope Land

The Sichuan Basin possesses a complete range of outcropped strata,with the most extensive exposure of sedimentary rocks,of which purple rocks cover the widest area.Purple rock is a typical representative of Mesozoic redbeds clastic sedimentary rocks in Sichuan Basin,which characterized by weathering quickly and easily.The soil from purple rocks was deeply affected by the parent materials.The purple rocks in the Sichuan Basin are mainly composed of mudstone,sandstone and sand-mudstone interbedded in the Cretaceous,Jurassic and Triassic systems and most of them are located on the gentle slope of low mountains or hills.Under the influence of terrain,the purple soil on slope land is formed from the parent rock under the acting force of water and gravity.In fact,the texture of horizons of a small number of purple soils on slope land was significantly different,but it was impossible to directly assess whether the obvious increase of clay content had been caused by sedimentation or the surface clay dispersed,migrated downward and deposited with the suspension.In general,when dividing the argic horizon,it is considered that the soil from sedimentary rocks with abundant rocks which exhibited differences in texture was caused by discontinuous lithology.Therefore,it was difficult to determine whether the soils from purple rock could be divided into argic horizon and Argosols.Assessing parent material uniformity is of great significance for the higher category in Chinese soil taxonomy.In addition,because purple soil is a lithologic soil deeply affected by parent material,it is generally considered that the differentiation between soil horizons is small,that is,the profile of purple soil is uniform.The slope deposit was formed by purple rock on slope land under the action of water scouring and carrying.Then the soils may be inhomogeneity in the parent material and profile under the acting force of water and gravity.At this time,assessing profile uniformity was helpful to identify the differences between the same and different profiles,which is of great significance for the lower category in Chinese soil taxonomy.Clarifying the relationship between the uniformity of parent material and profile can reveal the pedogenesis and evolution of purple soil.Based on this situation,this paper intends to study the profile uniformity of purple soil on slope land,which is divided into two dimensions: one is to assess whether the parent material of each horizon is the same in the deposition process according to the current soil properties;the other is to assess whether the profile is uniform according to the current characteristic in the soil forming process.In this paper,the morphological characteristics,physical and chemical properties(PCP),geochemical element characteristics(GE),generalizable horizon development indices(HDIs),the Ti/Zr ration,clay free particle size distribution(CF-PSD),uniformity value index(UV index)and weathering index(R)of 41 profiles form purple rock on slope land were systematically analyzed to assess the vertical uniformity of parent materials and profiles.Then the best and most convenient models were established based on the random forest algorithm for assessing the uniformity of the profiles.The main conclusion are as follows:(1)Among the purple soils on slope land,all profiles had little variation of Ti and Zr in the profiles,and the variation coefficients of Ti/Zr in the profile were less than <22%,which indicates that the parent material of purple soil on slope land was uniform in geochemical elements.While the particle size distribution of 34% of profiles had great variability from bottom to top,and the variation coefficients of clay-free sand and silt ratio in the profile were more than 25%.The absolute value of uniformity value indices of 56% of profiles were > 0.6.To sum up,63% of purple soils on slope land were assessed as lithologic discontinuity.Therefore,when purple soils on slope land were formed,the geochemical elements of parent material were uniform,while the heterogeneity of particle size of parent material caused by different surface erosion force of slope water.It was the main reason of lithologic discontinuity for purple soils on slope land.(2)Among the purple soils on slope land,80% of profiles derived from purple rocks had small bottom-up variability in general soil characterization,physical and chemical properties,HDIs and geochemical elements,stable accumulation and relatively uniform profile.20% of profiles had large internal color differences,HDIs suddenly changed along the vertical direction,and the physical and chemical properties and geochemical elements were greatly different in the principal component analysis(PCA)grouped by parent materials,which may be caused by discontinuous accumulation,being assessed as heterogeneous profile.The analysis showed that there are different degrees of correlation between the soil properties of purple soil on slope land,and the variation coefficients of each property in the profile were almost positively correlated.Therefore,it was reliable to assess uniformity by the variation coefficient of soil properties.The variation of geochemical elements of the profile and R horizon was greater than that of the profile.It could be inferred that,the complex parent material formed of abundant purple rocks during weathering process was one of the reasons for heterogeneous profile.Then,the reason of heterogeneous profile also were that,the soils inherited the discontinuous characteristics of parent material and some soils were differentiated in the process of soil formation.While the slope deposit and soil forming process can make 85% soils with lithologic discontinuity tend to be homogeneous.(3)The variation coefficients(CV)of soil properties were used as the input variables of the random forest model to assess profile uniformity derived from purple rocks on slope land.When the single variable was used for input variable,the Modle4(M4)which use the CV of CF-PSD had the best performance with the accuracy of 0.975.When the combined variable was used for input variable,the performance of the model using the CV of M13(PCP,GE,CF-PSD and R)as input variables was the best with the accuracy of 0.970.After ranking the relative importance of indicators of M4 and M13,the M16 which used the best model of combined variable to rank the CV of top five indicators of Al,Sa,clay,TP and Na as input variable had the best performance with accuracy of 0.971,having no significant difference with the best performance model(M4).Therefore,when assessing the profile uniformity of soil derived from purple rocks,the CV of CF-PSD is the best input variable of the model,it is most convenient to use the five indexes of CV of Al,Sa,clay,TP and Na as input variables.