Overcoming field barriers to restore dryland soils by cyanobacteria inoculation

Abstract Cyanobacteria inoculation to promote biocrust formation and improve soil properties has shown positive results in indoor studies, but limited success when applied under field conditions. Successful results of application of this technology in the field have been only found in desert sand dunes in China. Therefore, further development of this technique is necessary to extend its applicability to other soil types and regions. In this study, we inoculated a consortium of three N-fixing native cyanobacteria (Nostoc commune, Scytonema hyalinum and Tolypothrix distorta) on degraded soils from three semiarid study sites in south-eastern Spain with differing soil properties and soil development. After two years, chlorophyll a spectral absorption and albedo in inoculated and control plots were similar. Consequently, a second experiment was conducted to test the effectiveness of progressive cyanobacterial preacclimation before inoculation as well as the use of habitat amelioration techniques, consisting of covering the inoculated soils with a vegetal fiber mesh or a plastic grid, to improve cyanobacteria performance. Our results showed that: 1) hardening these cyanobacterial strains did not enhance their colonization capability, and 2) covering inoculated soils with a vegetal mesh did promote soil colonization by cyanobacteria, as shown by higher chlorophyll a soil content and Chla spectral absorption and lower albedo than in the uncovered plots. Moreover, it promoted the presence of more condensed, tightly-bound exopolysaccharides (EPS) and higher molecular weight molecules in the more soluble loosely-bound EPS fraction, both of which could be involved in the improvement of soil aggregation. Finally, higher abundance of xylose and galactose was also found in this treatment, likely indicating a greater development of the induced biocrusts. The results of this study show that direct soil inoculation with cyanobacteria, whether subjected to desiccation hardening or not, did not promote the artificial formation of biocrusts in the field. However, more positive results were found when the inoculated soils were covered with a vegetal mesh to help cyanobacteria cope with abiotic stress and soil erosion. Therefore, developing diversified efficient habitat amelioration strategies might be key in the successful application of this restoration technique in the field.