The mechanisms regulating the recovery of biological soil crusts (BSCs) due to the presence of nonphotosynthetic microbes were investigated using a soil scalping technique. Microscopic examinations identified the oglueo and oropeo action of bacteria and fungi at the initial stages of recovery of BSCs prior to the appearance of cyanobacteria. Extracellular polymeric substances (EPS) excreted by bacteria principally contained glucose and mannose. The optimum conditions for EPS production included the availability of glucose as the carbon source, the presence of CaCO3 (2g/L), KH2PO4 (0.3g/L), and MgSO4 (0.1g/L), a pH of 7 and incubation at 37 degrees C for 72h. Crust-forming tests in the laboratory and in the field demonstrated that inoculation of bare sand with oligotrophic bacteria was effective in accelerating the recovery of BSCs. The number of nonphotosynthetic microbes (especially actinomycetes and fungi) recorded in both the crust layer (0-2cm) and subsurface layer (2-5cm) was higher after 3 years than after 1 year. Microbial spatial variability of BSCs was related to nutrient status, especially available N.
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