Effects of soil erosion factors on cultivated-layer quality of slope farmland in purple hilly area

Abstract: Cultivated layer soil quality is highly sensitive to both soil erosion and artificial farming tillage in Purple Hilly Area. Soil erosion is a key factor that causes soil degradation and soil productivity instability of cultivated layer for slope farmland. Taking slope farmland in purple hilly area as an example, this paper has conducted cluster analysis on soil quality, permeability and cultivated layer type of slope farmland with different land productivity grades, and further discussed the effect of soil erosion on thickness of cultivated layer and the soil body configuration with reasonable cultivated layers of slope farmland. Results indicated that: (1) The soil quality of cultivated layer for purple soil slope farmland presented a moderate variability, the variation coefficients were 12.18% and 37.26% for the effective soil layer thickness (15-80 cm) and the cultivated layer thickness (15-25 cm), respectively, and the variation coefficient was 94.51% for the soil available phosphorus, indicating that the variability of soil physical properties was weaker than that of chemical properties. The cultivated layer configuration below the third grade of land productivity was almost full virtual or full real profile structures, and meanwhile, the production of slope farmland with the fifth grade of land productivity fell by about 50%. (2) There were significant (P<0.05) differences in soil physical properties, water-retention properties and tillage performances of cultivated layer at different slope position and different vertical depth of purple soil slope farmland, in which the soil bulk density showed upper slope > lower slope > middle slope, and soil shear strength showed upper slope > middle slope > lower slope, the soil stable infiltration rate showed the contrary order, and the soil penetration resistance was middle slope > upper slope > lower slope. The soil bulk density, shear strength and penetration resistance all presented bottom soil layer > subsoil layer > cultivated layer except soil infiltration; about 70% of the rain stored in 0-40 cm soil layer of slope farmland could be used by crops. (3) The average annual thinning of cultivated layer thickness varied between 1.04 and 3.04 mm for the moderate erosion degree of slope farmland. The reasonable cultivated layer of slope farmland could be established effectively by regulating such means as the minimum data set (MDS), which included soil layer thickness, cultivated layer thickness, soil bulk density, soil shear strength, soil organic matter and soil infiltration capacity for evaluating the cultivated layer soil quality of slope farmland, and moreover, the reasonable cultivated layer configuration overall should maintain the type of upper part loose and lower part tight, the cultivated layer thickness of 20-25 cm and the effective soil layer thickness of 50-60 cm. (4) Purple soil slope farmland could be divided into 4 kinds of cultivated layer types, whose obstacle cultivated layer was mainly manifested as the effective soil layer thickness restricted type (Class II), the cultivated layer thickness restricted type (Class III) and the soil nutrients restricted type (Class IV), accounting for 30%, 10% and 3% of total cultivated layer, respectively; so the regulating of reasonable cultivated layer of purple soil slope farmland should focus on effective soil layer thickness and cultivated layer thickness. These results can not only deepen the objective understanding of relationship between soil erosion and cultivated layer degradation but provide some essential parameters and technical supports for constructing reasonable cultivated layer of slope farmland in purple hilly area.