A cyanobacteria-based approach for rehabilitation of post-fire soils

Fire is one of the main environmental factors in Mediterranean ecosystems. Fire risk has increased during the second-half of the 20th century due to socioeconomic and political factors that have led to agricultural abandonment, large-scale afforestation, fuel accumulation, rural depopulation and expansion of urban areas. Fire causes important changes in soils such as a dramatic reduction of soil water availability and infiltration capacity, and an increase of soil water repellency, thus increasing runoff and erosion. Fire also modifies soil texture, aggregate stability, pH, organic matter and nutrient content and cation exchange capacity, and decreases diversity and abundance of soil biota and vegetation communities. Restoration of fire-affected areas aims at improving soil conditions and recovering the natural vegetation cover. The practice of cyanobacterisation (soil inoculation with cyanobacteria), which has been shown to be effective in increasing soil stability and fertility in natural and agricultural desert lands, could also provide good results in improving soil conditions in burnt soils. Cyanobacteria play multiple roles in the soil by increasing and organic matter and nutrients contents, soil structure and stability, water retention. The general objective of this study was to investigate the potential of cyanobacteria as inoculum for accelerating soil recovery in burned forest soils. With this aim, a mixture of two cyanobacterial strains, Scytonema javanicum and Phormidium ambiguum, was inoculated on burned soils from two temperate sites [Funchal (Madeira, Portugal), and Orentano (Lucca, Italy)] with different soil and climate characteristics, under laboratory conditions. Cyanobacteria effect on soil hydrophobicity, soil stability, and EPS content was monitored for 45 days. Fifteen days after the inoculation, the hydrophobicity, determined by the water drop penetration time (WDPT) test and the repellency index (RI), was decreased compared to the controls. The development of a cyanobacteria-induced biocrust was confirmed by the increase in chlorophyll a content and soil resistance over time. These changes were more significant in the soils from Funchal than in those from Orentano. Cyanobacteria inoculation also increased the exopolysaccharide content compared to non-inoculated soils. This study highlights the importance of the characteristics of the burnt soils in facilitating the improving action of cyanobacteria, and globally points to the potential of cyanobacterisation on the rapid stabilization of post-fire soils, which is one of the major concerns in managing burned areas immediately after fire occurrence to avoid net soil loss and major hydrogeological issues in the lowlands.