Role of biocrusts in soil rehabilitation in desert environments

The progression of desertification is an increasing environmental, social and economic emergency present on all the Continents. In order to remediate degraded soils and return to sustainable land use, an increase in soil organic matter (SOM) content and a more efficient and sustainable use of water resources are key factors to be considered. Soil quality depends on several physical, chemical, and biological properties, but generally soil microbial communities are key players in several important soil processes and both composition and activities of these communities can affect such processes. Many different approaches have been proposed for contrasting desertification and for the restoration of desert soils. Among them, an environmentally sustainable biotechnology based on the induction of the formation of biocrusts (BSCs) by inoculating cyanobacteria was recently proposed and applied in the field. Some of the documented direct effects of cyanobacteria inoculation are related to soil stabilization, enrichment in nutrients and increase in the capability to maintain a high moisture content. By inoculating cyanobacteria it is possible to induce the formation of BSCs, which are complex communities producing significant effects on the ecosystem with their onset. Different studies showed that filamentous strain Microcoleus vaginatus has a high tolerance to environmental stresses and a high ability to conglutinate sand grains starting from unconsolidated sandy soils in hyper-arid environments. In dryland soils, where C content is low, a huge share of the introduced C comes from microbial-secreted exopolysaccharides (EPSs), which represent the primary substrate respired after rainfall events by BSC microflora.. EPSs and their chemical and physical features strongly affect soil hydrological processes. Although the role of BSCs in water relations have still to be thoroughly clarified, in North American soils covered by natural crusts it was demonstrated that BSCs affect water sorptivity, while high molecular weight EPSs of induced BSCs on sandy soils were demonstrated to affect hydraulic conductivity and water capture from non-rainfall sources. Indeed, the abundance of EPSs was proved to be positively correlated to the water capture capability of induced BSCs, and when EPSs were artificially removed, the capability to capture water was not statistically different from that of bare sand. In this presentation a particular focus will be given on the role of the extracellular polysaccharidic matrix synthesized by cyanobacteria in giving the structure to natural or induced BSCs and to enhance their water trapping and retaining capability.