Degradation effect of soil erosion on tillage-layer quality of slope farmland and its evaluation trend

Soil erosion is the key driving force that causes tillage-layer quality degradation gradually and soil productivity variation precariously in sloping farmland. According to 2 functions of tillage-layer, erosion control and soil productivity, in this paper, we firstly focused on the ecological processes occurring in tillage-layer of farmland under the comprehensive interactions among soil erosion, soil and water conservation practices and agricultural activities at plot scale, and further summarized its influencing roads of soil erosion on tillage-layer quality. Results showed that: 1) Tillage-layer quality of sloping farmland was determined by the 2 ecological interaction process, rainfall erosion and tillage activities, and the temporal and spatial scales of these interaction on tillage-layer quality were very different. Soil properties functions indicating tillage-layer quality of slope farmland could be divided into such 4 types as water conservation, soil conservation, fertilizer conservation and production potential during a total agricultural production process. Crop-tillage coupling coordination could determine such characteristics of slope farmland as soil productivity, soil and water loss and the degradation direction & degree of tillage-layer caused by water erosion. 2) Tillage-layer quality was the characteristics of soil quality, its vertical combination along the active layer of crop root-system and underlying layer along the soil profile and the site conditions of sloping farmland. Tillage profile configuration of sloping farmland was the vertical distribution characteristics of soil texture, soil bulk density, soil porosity and soil mechanical resistance, so did its combination characteristics. The changes of tillage-layer quality of sloping farmland had obvious cumulative effects of water erosion on tillage disturbance. Degradation effects by water erosion on tillage-layer quality of sloping farmland were manifested in 3 aspects: deterioration of soil properties, deterioration of soil quality and decline of land productivity. The variation degree of soil physical properties was greater than that of chemical properties, and the decline of land productivity caused by runoff was greater than that caused by soil erosion. The change of crop yield had a significant hysteresis effect compared with soil quality degradation, meanwhile, soil permeability and soil erosion sensitivity had a direct correlation to the sustainable and stable productivity of sloping farmland. 3) In primary water erosion areas of China, an unified minimum data set of tillage-layer quality evaluation of sloping farmland should be set up aimed at the typical soil types and farming systems, which paid more close attention to the 2 functions of tillage-layer on erosion reduction and yield increase simultaneously. Such soil parameters as effective soil layer thickness, tillage layer thickness, soil bulk density, soil shear strength, soil organic matter and soil permeability could be included into the minimum data set for rational tillage-layer evaluation at plot scale. The time response characteristics of the minimum data set of tillage-layer quality should be fully taken into account in determining the threshold/suitable value. Rational tillage suitability of sloping farmland was divided into 5 grades, which were connected with soil erosion classification and cultivated land fertility classification. 4) Tillage-layer evaluation of slope farmland should focus on 3 aspects in the future, minimum data set of diagnosis index for tillage-layer quality, classification criteria of rational tillage threshold/suitable value and criterion of soil erosion control on sloping farmland. Accompanied by such normal indicators as soil erosion modulus, runoff coefficient and soil loss tolerance for protection of sloping farmland, the minimum data set index for diagnosing tillage-layer quality, as soil organic matter, soil infiltration, soil clay content could provide quantitatively a regional early-warning standards, which would benefit to more efficient soil and water loss control and realize sustainable utilization of sloping farmland These viewpoints were helpful in understanding the mechanism of degradation process caused by erosion of sloping farmland, and identifying quantitatively regulation approaches for rational cultivated-layer of sloping farmland, and also could provide some technical parameters for constructing rational tillage layer of slope farmland in water erosion area.