The extracellular polymeric matrix in biocrusts and its role in the improvement of soil properties

A large number of soil microbes, especially cyanobacteria, can produce extracellular polymeric substances (EPS) that can change soil properties in terms of sorptivity, water infiltration, soil matric potential, and also nutrient availability or organism colonization. Especially in degraded soils, the EPS effect can be decisive in fighting drought stress and can also be a technology for rehabilitating dry soils. EPS may effectively buffer crustal organisms against sudden dehydration, thus possibly enabling the preparation of dehydration-tolerance mechanisms. In complex and fragile systems such as biocrusts, the role of EPS in water retention may increase the chance of survival of crustal organisms in water-limited conditions [1]. Indeed, in recent years, inoculation-based techniques utilizing EPS producing cyanobacteria for the induction of biocrusts have proved to be a viable and sustainable pathway to increase soil biomass and fertility and to favor soil stabilization. The inoculation of cyanobacterial EPS alone, though increasing physical stability in sand crusts, did not sort most of the significant effects detected by inoculating the whole cultures. This pointed out the importance of the action of the cyanobacterial filaments as well, in soil conglomeration. The increase of inoculum amount coincided with the presence of a higher EPS content in the sand, which significantly contributes to sand fixation [2]. The evaluation of the optimal amount of biomass to apply is a key in the success of inoculum establishment on the sand, especially on coarse substrates, and the development of cyanobacterial biocrusts. Cyanobacteria inoculation increased the EPS content of the soil, regardless of the soil type [3]. The polymeric matrix formed by the inoculated cyanobacteria had different macromolecular and chemical characteristics in different soil types. This differential influence is likely to modulate changes in soil properties, nutrient cycling, and biocrust successional dynamics, eventually conditioning soil microbial community composition and activity and plant performance in dryland regions. A better understanding of how these polymers are produced can give fundamental insights into the development of efficient systems to assist soil restoration in water-limited environments.