EFFECT OF NOSTOC (CYANOBACTERIA) INOCULATION ON THE STRUCTURE AND STABILITY OF CLAY SOILS

The effect of Nostoc spp. (Cyanobacteria) inoculation on soil structur e 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 sail colonization and exopolysaccharide (EPS) production capacities, were inoculated in Petri dishes on the two clay soils sterilized by au toclaving, The soils, inoculated with an amount of cyanobacterial biom ass corresponding to 1.0 g dry wt. m(-2), were incubated under continu ous light at 27 degrees C for 3 months and periodically wetted using a pipette. The two strains showed different growth rates and EPS produc tion on both soils: KaS35 produced more biomass, while AfS49 produced more EPS. This different behavior was also documented by scanning elec tron microscope (SEM) observations. The effect of cyanobacterial inocu lation on soil structure resulted in the protection of soil porosity b y reducing the damaging effect of water addition. Indeed. the incidenc e of transmission pores in the inoculated soils (about 30%) was higher with respect to the control soils (about 5%). Data also showed the be ginning of a primary aggregation as a consequence of interaction betwe en the secreted EPS and the morphological units of the fine soil fract ion. However, no significant differences in water soil structure stabi lity were measured between inoculated and non-inoculated soils. In thi s paper the interactions between the EPS produced by the two strains a nd the clay aggregates are discussed in order to understand the role o f cyanobacterial inoculation in maintaining soil structure.