Innovative techniques to improve cyanobacterial survival and growth in inoculated dryland soils

Nowadays, land use change and the impacts of climate change are accelerating land degradation processes in drylands. These regions occupy around 40% of the Earth land’s surface and their extension is likely to represent around 45% by 2050. Biocrusts (complex communities formed by bacteria, cyanobacteria, microalgae, fungi, lichens and mosses which live in the uppermost layer of soil and can cover up to 70% of the interplant areas) play a decisive role in soil stabilization and fertility in these regions, so that they have been proposed as restoration agents in degraded dryland sites, where water scarcity and the harsh environmental conditions can hinder traditional restoration based on the use of vegetation establishment. Within the different biocrust-forming organisms, the use of cyanobacteria as a biotechnological tool to combat soil degradation, is gaining increasing importance. Cyanobacteria are the pioneer colonizers of terrestrial ecosystems, they are able to resist extreme environmental conditions, i.e. high temperatures, prolonged UV radiation and nutrients scarcity. At the same time, they improve physical-chemical properties of the soil by fixing carbon and many species also the atmospheric nitrogen and by producing exopolysaccharides that strongly increase soil stability and eventually creating a more favorable environment for colonization by other organisms. Despite several laboratory studies demonstrate the effectiveness of inoculating soil with cyanobacteria and their effect in increasing soil carbon and nutrient content, few field studies are available and many of them show a limited success probably because of the harsh environmental conditions that hamper an optimal growth. In the present work, soils collected from different ecosystems  in SE Spain were inoculated with a consortium of four native cyanobacteria species: Nostoc comune, Trichocoleus desertorum, Tolypothrix distorta and Leptolyngbia sp., and  different techniques to reduce abiotic stresses were tested in outdoors conditions: 1) cyanobacteria + soil covered with a mesh made of Stipa tenacissima, 2) cyanobacteria+ Plantago-based stabilizer amendment, and 3) cyanobacteria + sewage sludge (incorporated as an organic amendment) . The application of plant-based ameliorating strategies resulted in a higher chlorophyll a content, which reflects an improvement of cyanobacterial growth compared to the inoculation lacking the application of ameliorating techniques. The soil albedo also decreased due to surface darkening, thus also indicating a higher cyanobacterial growth in these treatments. Wind tunnel experiments also demonstrated a lower susceptibility to wind erosion in the cyanobacteria-inoculated soils combined with application of the plant mesh or the Plantago amendment. These results highlight the importance of using plant-based amelioration techniques to reduce abiotic stresses, especially in the early stages of soil colonization after cyanobacteria inoculation. Regarding the use of sewage sludge, it was demonstrated that their application at low doses improved cyanobacteria growth, which was reflected in an increase in chlorophyll a content as well as in a significant increase of aggregate stability and reduced soil susceptibility to wind erosion. This study shows promising results to enhance cyanobacterial growth and prevent cyanobacteria inoculum loss under natural conditions. Ongoing experiments will evaluate the effectiveness of these strategies under field conditions.