Effect of Nostoc (Cyanobacteria) inoculation on the structure and stability of clay soils

The effect of Nostoc spp. (Cyanobacteria) inoculation on soil structure was studied in two clay soils (Calanco and Biancana) originating by erosion processes from Pliocenic marine sediments of central Tuscany (Italy). Two axenic Nostoc strains, AfS49 and KaS35, selected for their soil colonization and exopolysaccharide (EPS) production capacities, were inoculated in Petri dishes on the two clay soils sterilized by autoclaving. The soils, inoculated with an amount of cyanobacterial biomass corresponding to 1.0 g dry wt. m^-2, were incubated under continuous light at 27°C for 3 months and periodically wetted using a pipette. The two strains showed different growth rates and EPS production on both soils: KaS35 produced more biomass, while AfS49 produced more EPS. This different behavior was also documented by scanning electron microscope (SEM) observations. The effect of cyanobacterial inoculation on soil structure resulted in the protection of soil porosity by reducing the damaging effect of water addition. Indeed, the incidence of transmission pores in the inoculated soils (about 30%) was higher with respect to the control soils (about 5%). Data also showed the beginning of a primary aggregation as a consequence of interaction between the secreted EPS and the morphological units of the fine soil fraction. However, no significant differences in water soil structure stability were measured between inoculated and non-inoculated soils. In this paper the interactions between the EPS produced by the two strains and the clay aggregates are discussed in order to understand the role of cyanobacterial inoculation in maintaining soil structure.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday