生物结皮粗糙特征——以古尔班通古特沙漠为例

The aerodynamic roughness length of biologicalsoil crusts:a case study of Gurbantunggut Desert

空气动力粗糙度可以反映地表气流与下垫面的相互作用。古尔班通古特沙漠是我国最大的固定、半固定沙漠,其间广泛分布的生物结皮在稳定地表和改善环境方面意义重大。对未经扰动的4种类型生物结皮进行表面微形态观察,并通过风洞实验确定动力粗糙度z0和摩阻风速u*,结果表明:(1)不同生物结皮类型,其组成和表面微形态等都具有明显差异。藻结皮以表面致密光滑为显著特征,由藻类分泌物和藻丝体粘结细粒物质所形成;地衣结皮表面藻类和真菌形成的叶状体匍匐沙面生长,呈现三维生长方式,形成有明显凹凸的壳状覆被;苔藓结皮以苔藓植物体密集丛生为特点,地上部分出现了茎叶分化,有一定的柔韧性。(2)就动力粗糙度的大小而言,是按地衣结皮>藻类-地衣结皮>苔藓结皮>藻结皮的顺序排列的,z0平均值依次为(6.589±0.850)mm、(4.179±0.239)mm、(2.542±0.357)mm和(0.393±0.220)mm,与定床裸沙面的(0.042±0.019)mm相比,生物结皮z0值提高了10—150倍。随着风速的增大z0值有所减小,其中以地衣结皮的减小趋势较为明显。(3)由风速廓线对比发现,4类生物结皮对气流阻滞作用的差异主要局限于4 cm以下的高度范围,风速越大这种差异也越大。各类生物结皮摩阻风速u*随风速增大而增大,其中藻结皮的增大速率明显低于其它3类结皮,说明藻结皮随风速增大的阻滞效应较其它3类结皮要差。(4)在净风条件下,地衣结皮具有最好的防风效果,其次为藻类-地衣结皮和苔藓结皮,藻结皮最差。当生物结皮破损后,床面结构和气流性质将发生变化,对空气动力学粗糙度和摩阻风速产生的影响将有待于进行更深入的研究。

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.