The soil dried layers under apple trees planation near Xi''an City
西安附近苹果林地的土壤干层

The ecological environments have been reconstructed at Chinese Loess Plateau, but little attention is paid to development of dried earth layers which may be disadvantage for planting trees at loess areas. To delimit distribution of the dried earth layers and to evaluate it suitable for planting trees or not, moisture contents of soil under apple trees were determined. Our soil samples were collected by the lightweight manpower's drilling at intervals of 10cm near two villages (Qujiangchi and Wujiafen) at the southern suburb of Xi'an, where ages of the apple tree plantations are 15, 10 and 6 years respectively. Soil layers of the profiles are Malan loess which consists of silt and clay. The soil moisture contents were calculated by electronic balance through weigh before and after the samples were dried in oven. The mean soil moisture contents for three drilling profiles are 10.1% above the depth of 1.9 meters, 9.3% between 2 and 3.5 meters and 13.0% between 3.5 and 6.0 meters under 15-year apple trees at Qujiangchi village. The mean moisture contents for two drilling profiles are 10.5%, 9.6% and 13.8% at the same depth ranges under 10-year apple trees, respectively. While, under 6-year apple trees, the mean soil moisture contents for two drilling profiles are 11.1%, 10.7% and 14.0% at the same depth ranges. Meanwhile, the soils under grass have higher moisture contents at the same depth ranges which are 11.6%, 13.1% and 14.3% from one drilling profile than those under apple trees.In general, the soil layers whose moisture content is lower than 10% are classified as dried layers. Our results indicate that the dried earth layers have been developed under both 15- and 10-year apple trees near Xi'an, whose distributions increase from south to north of the Loess Plateau. While soils under 6-year apple trees have been dried but no dried earth layers. However, though there are the dried earth layers near Xi'an, its dried intensity is relatively slight even after plantation for 15 years. The development of soil dried layer will decrease the normal growth of man-planted forest, and even cause trees to grow slowly and to wither in seriously affected districts. In such conditions, forestation will not bring economic and environmental benefit, rather than speed up soil degeneration through excessive consumption of soil moisture. We suggest that the development of the dried earth layers results from little precipitation, improper choice of tree species and high density of man-planted forest over the last decades. The key factors causing soil dried layers are great thickness and depth of membrane water zone under ground as well as small thickness of gravity water zone. Therefore, forestation should be performed scientifically and rationally, the loess area is suitable for planting trees if slowly growing trees are chosen with suitable tree planting intervals when the serious dried earth layers are avoided. Better growth of man-planted trees near Xi'an indicate