Abstract: To understand the effects of different vegetation cover on the bio-crust and its under-layer soil in Horqin Sand Land, the bio-crust and 0〖KG-*2〗-〖KG-*7〗5 cm soil samples in Artemisia frigida, Salix microstachya, Populus simonii, and Pinus sylvestris var. mongolica stands were collected, and their physical and chemical properties were studied. After fifteen years enclosure protection and vegetation establishment, the flowing sand dune in most natural and artificial vegetation sites was covered with bio-crusts. The coverage degree of the crust was 50%〖KG-*2〗-〖KG-*7〗80%, with a thickness of 1〖KG-*2〗-〖KG-*7〗2 cm. In S. microstachya stand, the crust thickness, hardness, and organic matter and nutrient contents were the highest, followed by in P. sylvestris var. mongolica stand, and in A. frigida and A. frigida stands. Moss crust had higher thickness, hardness, and organic matter and nutrient contents than lichen crust. Comparing with those in flowing sand land, the physical and chemical properties of under-layer soil in test stands were improved obviously, the improvement degree being higher under moss crust than under lichen crust, and higher in S. microstachya and P. sylvestris var. mongolica stands than in A. frigida and P. simonii stands. Vegetations with moderate height and rich branches and leaves were more helpful to the development of bio-crust and the improvement of under-layer soil, because they could reduce wind velocity and accumulate more dust and litters, and the development of moss crust was more favorable than lichen crust to the improvement of its under-layer soil.

Key words: biological soil crust, soil CO₂ flux, succession, hilly region of Loess Plateau.