Abstract: Investigation shows that in Shapotou region,the annual mean thickness of fallen dust(d<0. 063mm) is 0.144ram. Its suspended sal particles,calcium carbonates,trace elements and organic matters originated from lower plants,microbes and plant residues can promote the physical, chemical and biological changes of microenvironment. The amount and frequency of precipitation strongly affect the crust formation on sand surface and soil forming process. In sand dune with grain size of 0.250-0.125 mm,the permeating rate of precipitation is 1:7-1:10. Crust formation on sand surface can induce the change of moisture and heat status near earth surface,and hence, decrease the moisture content of sand layer to 2％ or below,which limits the function of vegetations in sand stabilization to certain extents.

Key words: Crust formation on sand surface, Microenvironment, Fallen dust, relative humidity., temperature, urban wetland, shape feature