关键词: 紫色土坡地, 退化阈值, 植物篱笆

Abstract: Mountains and hills accouns for 2/3 of the total area in China. Intense soil erosion of slopelands brings about serious consequences of environments, social and economic developments, being a restrictive factor for improving the living standards of the people and increasing of agricultural production. For maintaining the living space of human society, effective measurements should be taken for preventing land from further degradation, and rehabilitaing the degraded land, in order to enhance its productivity potential and promote regional sustainable development. The Three Gorge Areas are characterized by middle subtropical climate and the purple soils, with thin layer and sandclay texture are easy to be eroded. The efficient use of sloping farmland has been drawn more attention in the region. Different measures of land use, including engineering and biological ones were carried out for soil conservation and erosion control. Based on research results of the stationary experiments and observations on the purple soil slopelands in the Three Gorges Areas, the slopelands process, the measurements for restoration and management of degrading slopelands are discussed in the present paper. As concerns the degrading threshold of the purple soil slopelands, the threshold of slope gradients for the maximum intensity of surface erosion is 24°￣29°; the maximum diameter of soil particles eroded by water is less than 0 02 mm, including silt (0.02 mm￣0.002 mm) and clay (<0.002 mm); the selecting erosion of soil particles may cause soil organic matter loss, and the reduction of available N, P and K content in the soil surface. The loss of N, P and K is not associated with loss of water, but occurs in the process of selecting erosion of soil particles. The sediment filtering effect of living hedgerows has an additional advantage, preventing soil nutrient loss with runoff. For some soils, most nutrients are combined with small soil aggregated, while the content of nutrients in soluble state is low. Therefore the nutrient loss will be controlled if the sediment in runoff is kept on site. Silt and clay fractions have high proportions in purple soil particles and play the role of nutrient carriers. Experiment shows that nutrient content in sediments is high while that in runoff water is extremely scarce (Cai et al . 1996). Obersrvation from the field tests shows that the living hedgerows can decrease surface runoff enormously and reduce soil erosion obviously, as long as the living hedgerow forming an effective barrier. Because the slope length has been shortened by hedgerows, then runoff velocity of slopelands surface decreased, opportunity time of water infiltration prolonged, and runoff erosivity as well as transport capacity reduced after a continuous contour barrier being built. According to comparative studies of mechanical, agricultural and biological measurements in the study areas, the contour hedgerows of Leucaena leucacephala, Vitex negundo, Coriaria sinica, Hibiscus syriacus and Hemerocallis citrina form an earth bunds of 40 cm￣60 cm in hight within three years. It is quite possible to form hedgerow terraces or gentle sloping terraces in about 10 years. Green manure is the major harvests from the hedge plants used in the experiment site. Trials show that soil fertility, especially the physical and biochemical properties of soils have been improved with the regular application of cutoffs from the hedges. Three years after the implementation of the project, the soil organic matter and available nitrogen, phosphorus and potassium have increased obviously in trial plots of the experimental site. The purpose of ameliorating and utilizing slopelands is to prevent lands from soil erosion and deterioration of the physical and chemical properties of soils and improve the nutrient supply of soils. therefore it will be feasible to adopt slopelands agricultural techniques of contour hedgerow as the principal measure with some supplementary engineering means for restoration and management of the degrading purple soil slope.

Key words: purple soil slopelands, degrading threshold, hedgerow