Analysis on soil erodibility of different land use types in the Qingmuguan karst valley

The purpose of this work is to investigate the physicochemical properties, erodibility and influencing factors of soil in the karst areas of Chongqing City, so as to provide references for quantitative study of soil erosion and the formulation of soil and water conservation measures, and promote regional ecological protection and land use structure adjustment. The work area is in the Qingmuguan karst valley watershed of Chongqing City. This valley formed in the southward extension of the Wentangxia anticline in the Jinyun mountains, part of the parallel ridge-valley system in eastern Sichuan. Here a large area of Triassic carbonate rock is exposed. Under the action of long-term water current dissolution, a typical karst valley landscape of "one mountain, two ridges and one trough" developed. The geological strata are composed of carbonate rock(T1j、T2l) and clastic rock(T3xj). In this work, six land use types were selected as the objects, including cultivated land, coniferous and mixed conifer-broadleaf forest, bamboo forest, garden land, wild grassland and bare land. Surface soil (0-10 cm) and profile soil (0-60 cm) were collected to analyze material composition and organic carbon content, and the soil erodibility K was calculated by the EPIC model. Results show that, (1) The erodibility K value of topsoil in the study area is between 0.0371 and 0.0605, 0.0485 on average, with a median value 0.0475 and a variation coefficient 10.71%, and the skewness and the kurtosis are less than 1； (2) The topsoil is dominated by silt, attributed to silty clay loam. The organic matter content is between 13.98 and 52.24 g.kg-1, with an average value of 29.20 g.kg-1.The K value of topsoil erodibility is negatively correlated with sand content and carbon content（P <0.01）, positively correlated with silt content（P <0.01）,and negatively correlated with clay content（P<0.05）significantly； (3) The topsoil erodibility K values of different land use types in order are: bare land(0.0583)>cultivated land (0.0534)>garden land(0.0483)>wild grassland(0.0478) >bamboo forest(0.0469) >mixed conifer-broadleaf forest(0.0427),with prominent differences (P < 0.05), and the coefficients of variation range from 2.7% to 6.1%. Bare land and cultivated land that are strongly influenced by human activities are the primary sources of sedimentation in the region.Vegetation restoration can effectively improve soil erosion resistance；(4)For different land use types, the average soil erodibility K value in the soil profile of cultivated land (0.0563) is significantly higher than that of wild grassland (0.0516) and forest land (0.0481) (P<0.05). While the difference of soil erodibility K values between wild grassland and forest land is not obvious.In soil profiles,the K shows relatively small values with great variations in depth range 0-35 cm,and increases with depth in 35-60 cm below the surface.