| The research on the physical and mechanical mechanism of soil erosion of forest in Jinyun Mountain of Chongqing City was conducted, which are associated with the National Natural Science Fund project "The Relationship Research between Soil Structure Characteristics and Soil Erosion of the Three Gorges Reservoir Area Based on Fractal Theory ". The purpose of this paper is to provide scientific evidence for soil erosion research and vegetation construction and sustainable ecological environment development of the Three Gorges reservoir area.Soil structure of four typical forest lands (mixed Pinus massoniana-broadleaf forest, evergreen broad-leaved forest, Phyllostachys pubescens forest and shrub forest) and the agricultural land in Jinyun Mountain, was comprehensively and quantitatively studied by the method of the fractal theory for the first time. The results are as follows:particle size fractal dimension (PSD) of mixed forest land was optimal (2.936), the one of Phyllostachys pubescens forest land was worst (2.908); micro-aggregate size fractal dimension (MASD) optimal for shrub forest land (2.582), the one of agricultural land was worst (2.718); aggregate size fractal dimension (ASD) optimal for mixed forest land (2.706), the one of agricultural land was worst (2.762); pore fractal dimension (PFD) for shrub forest land optimal (2.773), the one of Phyllostachys pubescens forest land was worst (2.466). With the method of nonlinear predictive indicators, the relation between soil properties and soil structure fractal dimension was fitted by the model ofγ=αDjβ1Dwβ2Dtβ3Dkβ4. PSD's impact on the model was greater than MASD and PFD, but less than ASD.The physical and mechanical mechanism of soil erosion of forest was analysized and evaluated by the methods of statistical analysis, canonical correlation analysis, grey reational analysis, stepwise regression, principal component analysis, fuzzy-clustering analysis and other methods, the results were as follows:Impermeability of shrub forest land was the strongest, while agricultural land was worst. Initial infiltration rate was mainly affected by PSD and ASD, followed by the MASD and PFD. Effect on steady infiltration rate with ASD and PSD was greater than PFD, the least was MASD. Infiltration rate was mainly affected by pore (aperture> 1.5 mm), followed by the pore (aperture was amang 0.4 to 0.9 mm), the least was pore (aperture< 0.4 mm). Anti-dispersion of shrub forest land against was the strongest (78.4%) and agricultural land was the worst (42%). Anti-dispersion dynamic changes with time was fitted by the model of s=at2+bt+c. The best indicator of anti-erodibility evaluation was the disarrangement rate of aggregate. Anti-erodibility of shrub forest land was the strongest and Phyllostachys pubescens forest land was the worst. The order of soil shear strength was mixed forest land> evergreen broadleaf forest> shrub forest> Phyllostachys pubescens forest> agricultural land, which was mainly affected by MASD, followed by bulk density, dry density, cohesion and the hardness. The index of soil anti-scouribility had the great remarkable power function with time and its relation with slope was fitted by the model of K= 0.0755e 0.0755x.Soil anti-scouribility of mixed forest land was the strongest, Phyllostachys pubescens forest land was the weakest, it was mainly affected by PFD, ASD, macropore radius and steady infiltration rate, followed by macropore section area ratio, structure index and organic matter. Critical shear stress of mixed forest land was the largest (1.257 N/m2), agricultural land was the smallest (0.824 N/m2), the critical shear stress depended mainly on soil shear strength.Impermeability, anti-erodibility, shear strength and anti-scouribility reflect the physical and mechanical mechanisms of soil erosion, soil erosion which are important indicators to evaluate the degree of soil erosion. Then, the method of analytic hierarchy process was applied to evaluate the degree of soil erosion under the typical forest land, the result is as follows:shrub forest (1.260)> mixed forest land (1.169)> evergreen broadleaf forest (1.106)> Phyllostachys pubescens forest land (1.066)> agricultural land (1.042). Therefore, the resistance to soil erosion of shrub forest land is the strongest, and agricultural land was the worst. Forest vegetation can improve soil anti-erosion. The physical and mechanical mechanisms of soil erosion in Jinyun Mountain was studied in-depth, the evaluation index system of soil anti-erosion was proposed, and soil anti-erosion properties of different typical forest lands were obtained. It fills in the blanks of analysis and evaluation of physical and mechanical system of soil anti-erosion in Jinyun Mountain.
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