Abstract:The Gurbantunggut Desert, the largest fixed and semi-fixed desert in China, is covered by well-developed biological soil crust, formed by different combination of mosses, lichens, algae, fungi, cyanobacteria and bacteria. Biologically, this kind of crust differs a lot from physical soil crust in physical and chemical properties, and it serves as one of the biological factors contributing to the sand fixation in the Gurbantunggut Desert. Additionally, biological soil crust plays a significant role in desert ecosystem, involving in the process of formation, stabilizing and fertilizing soil, preventing erosion by water or wind, increasing the possibility of vascular plants colonization, and being responsible for the stabilization of sand dunes. In recent years, researchers have focused on the interactions between surface winds and crust surfaces, which can be characterized by aerodynamic roughness length. In this paper, an experimental study was carried out to investigate the microstructures of four different types of biological soil crust surface through microscopic examination. The soil samples were collected using the dustpan-like iron boxes (30cm×20cm×10cm), which were pushed into the ground to collect undisturbed soil samples covered by representative biological soil crust (moss crust, lichen crust, algal crust and algal-lichen crust respectively) and bare sand for control. The aerodynamic roughness length (z0) and friction velocity (u*) were determined in a wind tunnel. The results indicated that: (1) the species composition and surface features of the biological soil crust varied with their development stages. Algal crust, with compact and glossy surface, was an intricate network of filamentous cyanobacteria and exopolysaccharides that binds and entraps sand grains and conglutinated fine particles. Lichen crust had a rugose and pinnacled surface appearance, on which the thallus generated by algae, cyanobacteria and fungi creeped along soil surface and manifested as a three dimensional leaflike cover. Moss crust surface was quite flexible and characterized by densely and caespitosely moss plants that differentiates into stems and leaves. (2) The mean aerodynamic roughness length of lichen crust, algal-lichen crust, moss crust and algal crust are (6.589±0.850)mm, (4.179±0.239)mm, (2.542±0.357)mm and (0.393±0.220)mm, respectively. The values tended to be 10-150 times larger than that of fixed sandy bed. The aerodynamic roughness lengths of all crusts surfaces decreased with free-stream wind velocity. (3) From wind velocity profile we concluded that the block effects of crusts on air flow are mainly limited to no more than 4cm height from bed. The friction velocity of crusts increased with wind velocity, which means that the block effect of crusts on air flow tended to be better with increasing of wind speed. (4) Generally speaking, under clean wind condition the lichen crust had the best effect to control wind blow, followed by algal-lichen crusts and moss crust, while algal crusts had the lowest effect. Disturbances of the crusts will modify the surface features and wind-sand flow, and their effects on aerodynamic roughness and friction velocity need further investigated in future. This study will help us to improve our understanding of the aerodynamic roughness length of biological soil crust in arid areas and provide valuable scientific information that can be utilized in policy-making for the management of desert ecosystem.