Biological soil crusts are very sensitive to human-induced disturbances and are in a degraded state in many areas throughout their range. Given their importance in the functioning of arid and semiarid ecosystems, restoring these crusts may contribute to the recovery of ecosystem functionality in degraded areas. We conducted a factorial microcosm experiment to evaluate the effects of inoculation type (discrete fragments vs slurry), fertilization (control vs addition of composted sewage sludge), and watering frequency (two vs five times per week) on the cyanobacterial composition, nitrogen fixation, chlorophyll content, and net CO2 exchange rate of biological soil crusts inoculated on a semiarid degraded soil from SE Spain. Six months after the inoculation, the highest rates of nitrogen fixation and chlorophyll a content were found when the biological crusts were inoculated as slurry, composted sewage sludge was added, and the microcosms were watered five times per week. Net CO2 exchange rate increased when biological crusts were inoculated as slurry and the microcosms were watered five times per week. Denaturing gradient gel electrophoresis fingerprints and phylogenetic analyses indicated that most of the cyanobacterial species already present in the inoculated crust had the capability to spread and colonize the surface of the surrounding soil. These analyses showed that cyanobacterial communities were less diverse when the microcosms were watered five times per week, and that watering frequency (followed in importance by the addition of composted sewage sludge and inoculation type) was the treatment that most strongly influenced their composition. Our results suggest that the inoculation of biological soil crusts in the form of slurry combined with the addition of composted sewage sludge could be a suitable technique to accelerate the recovery of the composition and functioning of biological soil crusts in drylands.

Biological soil crusts (BSC) play a major role in water and nutrient fluxes in semi-arid and arid areas, affecting the establishment of vascular plants and contributing to the spatial arrangement of vegetated and open areas. However, little is known regarding their effects on the performance of extant vegetation. By using experimental manipulations (surface soil cutting and herbiciding), we evaluated the effect of the physical structure and the biotic component of smooth biological soil crusts on soil moisture dynamics, and on the nutrient and water status, growth rate, and reproductive effort of Stipa tenacissima tussocks in a semi-arid steppe. Soil moisture content was weakly reduced after cutting the soil surface and was not affected by herbicide application. Cutting and herbiciding the biological soil crust had no effect on most morpho-functional and reproductive traits measured in S. tenacissima tussocks. The integrated water use efficiency of this species, as measured by 13 C natural enrichment, decreased when the biotic community of biological soil crusts was killed with herbicide. In the S. tenacissima steppe studied, killing the BSC and breaking the continuity of the structural crust had a weak effect on the short-term performance of this species, but our results suggest that BSC exert a control on slope hydrology beyond that provided by physical soil crusts.