ABSTRACT
Biological soil crusts are soil surface communities of biota that live within or immediately on top of the uppermost millimeters of soil. They consist of cyanobacteria, algae, mosses, microfungi, and lichens. Cyanobacterial and microfungal filaments, rhizinae and rhizomorphs of lichens, and rhizinae and protonemata of bryophytes weave throughout the top few millimeters of soil, gluing loose soil particles together (Figure 6.1). The intimate association between soil particles and organisms forms a more or less coherent crust. A quantitative estimate of global biological crust cover is difficult to obtain and not yet available, but the worldwide coverage of the terrestrial surface by soil crusts is very high. In arid and semiarid areas, biological soil crusts may constitute up to or more than 70% of the living cover.
