Anti-erodibility Of Soils Developed From Suining Formation In The Hilly Area Of Central Sichuan Basin

Soil anti-erodibility is an important parameter for assessing the erosion resistance of soils,and is a hot spot in soil erosion research.The hilly area in the middle of Sichuan Basin belongs to the southwest purple soil water and soil control area,and the purple soil of Suining Formation is widely exposed in the area.The purple mud shale area of Suining Formation,which has a single soft lithology and a high soil forming rate,has a high land reclamation rate,a wide distribution of sloping farmland and a variety of land use types.The water and soil loss in this area is particularly serious,and it has also become an important sediment source area in the middle and lower reaches of the Yangtze River.The study on the anti-erodibility of purple soil in Suining Formation is of great significance to the prevention and control of purple soil erosion and land use adjustment.In this paper,the purple soil with Jurassic Suining Formation rocks as parent material in the hilly area of the central Sichuan Basin is taken as the research object.Soil samples of six typical land use types（trees,shrubs,wastelands,grasslands,orchards and drylands）in the purple soil area of Suining Formation are collected to explore the soil anti-erodibility and discuss the factors influencing the anti-erodibility of purple soils in Suining Group.Based on the physical and chemical properties of purple soil,soil anti-erodibility indexes were selected,and the differences of anti-erodibility of purple soil under different land use patterns and their causes were studied.The main findings of this study are as follows.（1）The purple soil of the Suining Group in the study area has a bulk weight of 1.52g/cm³,a water content of 21.35%,a total porosity of 43.77%and an overall neutral or weakly acidic soil.Soil particle size is dominated by powder content,with the lowest sand content and severe soil claying.The organic matter content ranged from a minimum of0.02%to a maximum of 5.14%,with a mean value of 1.84%,and 93%of the soil samples had a critical level of soil organic matter（CLOM）value of less than 5%.The soil has a high bulk weight,a higher water content and a low total porosity,which makes the soil less aerated and less infiltratable and prone to surface runoff.The low organic matter content makes it difficult for the soil to form a hydrophobic coating and the soil structure is easily damaged.The purple soils of the Suining Group therefore have a high potential for soil erosion.（2）The soil anti-erodibility of purple soils in the Suining Group by different land use methods was as follows:trees＞shrubs＞barren land＞grassland＞orchards＞dry land.There were no significant differences in clay,powder and fine clay grains among the six land types,while sand grains,physical clay grains and powder clay content were significantly different.The sand grains in the dryland were significantly lower than those in the abandoned land,while the powdered clay content was significantly higher than that in the abandoned land,and the soil particles of physical clay grains were also significantly higher than those in the orchard and abandoned land.The organic matter of dryland was lower than that of abandoned land and trees.The organic matter content of treesvitae was significantly higher than that of orchard and abandoned land.The percentage content of water-stable agglomerates＞0.25mm in trees was significantly higher than in orchards and drylands.The destruction rate of aggregates＞0.5mm in dryland was significantly higher than that in trees and orchards.The weight diameter of air-dried agglomerates was significantly lower in drylands and trees than in shrubs,orchards and abandoned land.Soil particles in drylands became finer under human disturbance,and agglomerate stability was the worst.After abandonment of cultivated land,human disturbance decreased,coarse particle content increased,and agglomerate stability was enhanced.（3）The best indicators for evaluating the resistance of purple soils of the Suining Formation are clay grain,clay ratio（CR）,soil agglomeration status,soil agglomeration,soil dispersion rate,＞0.25mm water-stable aggregates(WSA_＞0.25mm)、＞0.5mm water-stable aggregates(WSA_＞0.5mm)、Average weight diameter of water-stable aggregate(MWD_wet)、＞0.25mm percentage of aggregate destruction(PAD_＞0.25mm)and＞0.5mm percentage of aggregate destruction(PAD_＞0.5mm).These 10 indicators can better reflect the resistivity of purple soils of the Suining Formation in the study area.Based on these indexes,a comprehensive evaluation model of corrosion resistance of purple soil in Suining Formation is constructed.（4）The main factors affecting the soil anti-erodibility of purple soils in the Suining Formation in the study area include:soil water content,soil particle size composition and microagglomerates,water-stable agglomerates,soil resistance index,soil organic matter and critical levels of soil organic matter.Soil moisture content influences the degree of water stability and damage of agglomerates＞0.25mm and＞0.5mm.Soil anti-erodibility increased with increasing clay,coarse sand,and microagglomerates of 0.02 to 0.05 mm size,while it decreased with increasing chalk,very fine sand,fine sand,and microagglomerates of 0.05 to 0.1mm size.the content of water-stable soil agglomerates of 1 to 0.5 mm,0.5mm to 0.25mm,and＞5mm particle size and the average water-stable agglomerate weight diameter influence the agglomeration status of the soil.As soil organic matter increases,the content of water-stable agglomerates＞0.25mm and＞0.5mm also increases,as does the soil erosion resistance index,the greater the soil erosion resistance index,the greater the soil erosion resistance.Soil organic matter affects not only the agglomeration status,agglomeration and dispersion rate of the soil,but also the content of water-stable agglomerates＞0.25mm and the rate of structural damage.