Abstract: The mulching effect test was carried out in middle desert zone of Hexi Corridor with precipitation 113.4 mm, drought degree 5.1. A 4 year old Pyrus yard with stock of Pyrus betulaefolia Bage was selected to be covered with wheat straw of 1 m width and 15~20 cm thickness in May of 1997, May and July of 1998. The check plot was bare land. The measured contents included salt content, moisture (0~60 cm), temperature (0~25 cm), organic matter (0~30 cm), weeds, roots distribution and growth amount of one year old twig. When covered wheat straw in raw of Pyrus yard for 18 months, the contents of K⁺+Na⁺, Ca²⁺ and SO₄^2- of 0~30 cm soil reduced; the salt contents of 0~10 cm reduced by 67% and that of 30~60 cm increased by 0.06%; the salt content of 10~30 cm soil did not change. These showd that the covering wheat straw in raw of Pyrus yard are advantageous for draining salt of 0~30 cm soil, and reducing gathering amount of salt at surface of land. When soil moisture is lower, the change type of salt content of 0~10 cm soil is similar to bare land with season changing, but change range is lower. When soil moisture is higher, salt content of soil changes negatively with temperature variation. In the season with higher temperature, the change range of soil temperature in a day is reduced by covering straw in raw of Pyrus yard. When temperature is lower in autumn, the temperature of soil covered by straw increases and then goes down slowly. The amount of organic matter increases by covering wheat straw. But with prolonged time, the increase ratio of organic matter in soil relatively reduces. The increasing amount of organic matter after 4 months from being covered straw in raw of Pyrus yard is 5 times as much as the increasing amount of organic matter after 18 months, and 138 times as much as bare land's. The time for covering straw in Hexi corridor may choose May to early August. In the early period of covering straw, amount of annual weeds, wheat is a main weed, in raw increase. With prolonged time of covering straw, amount of annual weeds reduce. After 18 months of covering straw, the amount of annual and perennial weeds is 8.6 and 77.8 percent of bare land's. The covering wheat straw in raw of Pyrus yard is beneficial to the growth of top roots of Pyrus: The amount of roots of Pyrus (< 2 mm) in 0~10 cm and 0~60 cm soil is 37.5 and 2.67 times to bare land's. The growth amount of one year old twig of Pyrus tree on bare land is 87.17 percent of tree's on land with straw. When air temperature is higher, 0~60 cm layer of soil run away water in wheat straw coverd land, but take in water in bare land, and the reason need further studying.

Key words: arid area, inter trees, sod mulch, effect