| The moisture content is the most important factor that restrictsnorthwest ecological environment construction and the important factor that restricts thedevelopment of the agriculture and the forestry. Because in the recent decades theconstructed artificial forest often cannot achieve the anticipated goal for being lack ofmoisture, so the research to the soil - water environment in the artificial forest has theextremely vital significance. Through determining the soil moisture in different artificialforest from the surface to below 6 meters in Xi'an area of the most south of the loessplateau, the author has studied some problems such as in local area the soil moisturecondition in the artificial forest in the normal year and the abundant water year, therestoration of water in the soil dried layer, the distribution of the soil dried layer, themain reason of development and so on in this article. The samples studied are gotthrough light-duty manpower. The depth of the hole is 6 meters, and the distance is 10centimeters. In every hole 60 samples are got. The determination of the soil moisturecontent uses the drying weight method which is considered accurate and reliable. Thespots include Wujiafen in the south suburb of Xi'an, Qujiang Pond, Baqiao districtShaoping, Gaoling tuan village, Iintong Xiekou, Baimiao and so on. The trees in thechosen forest land are the pear woods, the apricot woods, the apple forest and the planetree forest, the Yang woods, the cedar forest, the Chinese scholar tree woods and so on.In addition, for the contrast, the soil moisture contents in the weed and the farmlandhave also been determined. Because annual means precipitation in 2003 is 883millimeters which is 283 millimeters higher than the normal yearly averageprecipitation (600 millimeters), and the year of 2003 belongs to the abundant water year,so taking May of 2003 as the boundary, the author takes the soil moisture content fromthe early 2002 to the early May of 2003 as that in normal years and takes that after thatperiod as that in the abundant water years. Through the determination and analysis ofthe soil water content in different artificial forests, the follow material and conclusionare got.(1) The predecessors have researched that the soil dried layer exists widely in Yan'an and the northern area of Yan'an, but the distribution in the south area of Yan'an and whether the soil dried layer in Guanzhong area exists or not have not been reported. In Xi'an area field stable water capacity is approximately 12%, therefore the soil layer of below about 150 centimeters and the water content lower than 12% all may be regarded as the soil dried layer. Under the normal precipitation condition, the water content in the layer of about 150-400 centimeters in the plane tree forest in Gaoling is 9.3%, that of 200-400 centimeters in the apple forest of 15 ages, 10 ages and 6 ages is separately 9.2%, 9.4% and 10.5% in Qujiang Pond and Wujiafen, that of 150-400 centimeters in the plane tree forest of 16 ages and in the Yang woods of 15 ages in Lintong Xiekou is separately 9.3% and 9.2%, and that of 150-400 centimeters in the Chinese parasol trees of 16 ages and in the Yang woods of 14 ages in Lintong Baimiao is separately 9.0% and 8.7%. The above data indicate that in Xi'an area, under the normal precipitation condition, the water content in the layer of 1.5-4.0 meters all kinds of artificial forests is generally lower than 10% and the soil dried layer develops apparently. From this the author is sure that the distributing scope of soil dried layer in the planted forest has already arrived to Xi'an area of the most south area of the loess plateau. The new conclusion is significant to reveal the cause of the soil dried layer in the loess plateau and the forest plantation.(2) The predecessors have realized that the factors such as climate, terrain, tree seed choice, planted way and so on can have the influence on the soil moisture content. But so far the important function of the film water has not been found. The author's study indicates that in the loess plateau the film water belt's little burial depth determined by little precipitation is the main reason for the soil dried layer's development. In loess plateau area the depth of the gravitational water belt is about 2 meters, below which there is the film water belt. The supply and migration of the film water are very slow, and the content of that is low considerably. If the crowded plant root system extends to this layer and absorbs moisture content, the moisture content is consumed very quickly, which cause the soil dried layer's development. The film water not promptly supplementing the short moisture content is the most direct and important factor of the soil dried layer's development.(3) In Xi'an area the soil moisture content in different artificial forest differs great. The research indicated that the soil moisture content of the economic fruit tree forestsuch as pear woods is smaller than the general ecology forest. But in the ecology forest the soil moisture content of various ecology forests is different. In WuJiafen the soil moisture contents from surface to 6 meters under surface in the plane tree forest is biggest, next is the Yang woods, and finally is the cedar forest in three seasons. In Baqiao district, the average soil water content in the plane tree forest and the mixed forest of the pane tree and the Yang tree which are close to each other in three seasons in 2004 is separately 15.0% and 24.1%, 13.7% and 20.5%, 15.4% and 22.1%. This is because the topography of the former is apparently lower than that of the latter. The pear woods in Gaoling grow fast and are planted densely, which causes the soil water content is apparently lower than that of other seeds.(4) In Xi'an area the soil moisture content in artificial forest varies remarkably with seasons changing. In all forest except the plane forest the soil moisture content is highest in spring, next is in summer, and in October of autumn the water content falls to the minimum value. This is because from April to October the trees grow fast, and the leaf surface transpiration and the ground evaporation are both strong, so the soil water content of the planted forest starts to reduce gradually from April, and the soil moisture content falls to a stable low value in autumn through much water consumption in the whole growth period. When in this year the normal precipitation is insufficient for the plant's growth, the soil then can appear the drying phenomenon.(5) That the soil dried layer is difficult to restore is generally approved, but the author's research indicates that in Xi'an area, influenced by the rich precipitation in May, 2003, the average water content in the soil dried layer of 1.5-4.0 meters in all kinds of artificial forest is almost between 15% and 20%, and the maximum is about 23%. Through the water supply of 10 months, from June, 2003 to April, 2004, the dried layer has all disappeared. This indicates that under the condition of precipitation remarkably increasing, the soil dried layer may restore in Xi'an area, and the depth that can be restored may reach 4 meters under surface and even 5 or 6 meters.(6) For the main cause of the soil drying layer development, we can make sure that in loess plateau the forest plantation and eliminating the soil dried layer are impossible, but we can take the scientific planting ways and the engineering auxiliary measures to extenuate the strength and damage of the development of the soil dried layer. In the plantation process the planted density should be reduced, the seeds that little water consumption and slow growth should be chosen, the savanna, the grass and the bushes...
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