黄土丘陵区人工林草地的土壤水分生态环境

黄土丘陵区的土壤水分状况属蒸发自成型水分状况，伴随有土层的干燥。在人工林草植被作用下，土层将会进一步向干燥化方向发展，从而形成“复合干层”或“重叠干层”。土壤水分的短期或长期补偿，都难以使其土壤湿度恢复到田间持水量状态。草原地带应以建造适应性强的灌木为主，森林草原地带着重建造防护-薪炭互补的防护林体系。改撂荒地为人工草地，是经济利用土壤水分资源、提高土地生产率的优化措施。

Soil Water Ecological Environment on the Man-made Woodland and Grassland in Loess Hilly Region

In semiarid loess hilly region ground-water table is present at a very deep depth. Thus water requirement for growth of plant can not get supplement from ground water. The water in soils within this region is mostly in status of suspended water. Soil water regime was considered as an automorphic -evaporative type, of which was followed by aridization of soil layer. Background of soil water in loess hilly region exist in four characteristics, they were: (1) obvious difference of water content in soils on differental landform; (2)very active upward evaporation ability of soil water; (3) low water content under field steady moisture;(4) existence of water storage difficulty in deep soil layer and weakness of regulation ability of soil water for growth of plant. Therefore under the automorphic -evaporative type of soil water regime as a result of action of manmade vegetation the aridization of soil layer is further intensified, of that formed "complex dry soil layer" or" overlapping dry soil layer". As the dried soil layer due to the absorption by roots of trees and shrubs is formed, its soil moisture is difficultly restituted to field capacity through the short-term or long-term compensation of rain water. Therefore forest planting and grass cultivating are situated under a unfavourable soil water ecological environment. According to bioclimatic circumstance the northwestern part of loess hilly region is located in the steppe zone; its centre and southern part-the forest steppe zone. Thus the climate changes increasingly arid to the west. The soil conditions for plantation as well changes unfavourably in this direction. In steppe zone shrubs which has best adaptation to arid circumstance should be planted. In the forest steepe zone it may be recommended to use complex porest type to protect against soil erosion and to solve the problem of fuel shortage. For raising efficiency of soil water use in the loess hilly region the wasteland is best changed to man-made grassland. This measure is an optimum method of efficiency useing soil water resources. At the same time it may be to improve fertility of soil and to increase grass yield.