The microstructure and formation of biological soil crusts in their early developmental stage

The biological soil crust serves as one of the biological factors contributing to the sand fixation in the Gurbantunggut Desert, the largest fixed and semi-fixed desert in China. This study was conducted to investigate the microstructure and formation of biological soil crusts which develop as a result of occurrence of cryptogams. One year after removal of biological soil crusts, the exposed surface could be fixed by bacteria, which make sand particles cohere by exopolysaccharides. The exopolysaccharides were mainly composed of glucose, mannitol, arabinose and galactose. The intension of pressure for this kind of crust is 13.42±1.38 Pa. After four-year recovery of the exposed sandy surface, the biological soil crust resulting from the colonization of soil surface by communities of filamentous cyanobacteria were mainly dominated byMicrocoleus, which occurred as a cluster of filaments surrounded by a gelatinous sheath. At this developmental stage, the main contributors for sand fixation were changed from bacteria to filamentous cyanobacteria. Microscopic examination of this kind of crust revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions, which binds and entraps mineral particles and finer particles on the filament surface. These effects enhance soil cohesion and resistance to erosion. The intension of pressure for this kind of crust is 32.53±3.08 Pa.