Establishing rates of lateral expansion of cyanobacterial biological soil crusts for optimal restoration

Aims Biocrusts that form on topsoils contribute ecosystem services to drylands, and their loss under anthropogenic pressure has negative ecological consequences. Therefore, development of biocrust inoculation technology for restoration is of interest. This requires knowledge of biocrust growth and dispersal. To contribute to this, we determined the speed at which biocrusts expand laterally based on the self-propelled motility of cyanobacteria. Methodology We inoculated sterile soil with natural biocrusts and incubated them over a year in a greenhouse under conditions mimicking local precipitation, monitoring the crust's lateral expansion using time-course photography, chlorophyll a content, and microscopic inspection. Concurrent uninoculated controls served to monitor, and discount, natural inoculation by aeolian propagules. Results While the expansion front was highly variable in space, biocrusts expanded in the order of 2 cm month(-1), but only in seasons with moderate temperatures (Spring and Fall). Microcoleus vaginatus, Microcoleus steenstrupii, and Scytonema spp. advanced at averages of 1 cm month(-1), the crust advance front being preferentially driven by specialized propagules (hormogonia). These rates are within expectations based on instantaneous gliding motility speeds of cyanobacteria. Conclusions Based on the expansion capability of biocrusts during growth seasons, greenhouse inoculum units can be optimally spaced to fill 4-8 cm gaps.