Bacterial exo-polysaccharides: a biological tool for the reclamation of salt-affected soils

International audienceAccumulation of salts on soil surface and in the root zone damages physico-chemical and biological properties of salt-affected soils. Exo-polysaccharides (EPS), the polymers of monosaccharides, are synthesised and released in soil by microorganisms inhabiting rhizosphere, roots of the plants and the decomposing organic residues. The bacterial EPS are involved in formation and stability of soil micro-aggregates, a factor that ensures fertility of the cultivated soils. The rhizosheaths formed around roots by bacterial EPS contribute to build up soil physical structures, regulate nutrients and water flow from rhizosphere soil to the plants, promote growth and protect the roots against pathogens. Thus, the bacterial EPS are directly and indirectly involved in and impacted both physico-chemical soil characteristics and growth of the plants. However, the role of the bacterial EPS in improving soil fertility and interaction with constituents of the salt-affected soils has rarely been explored. Research studies, therefore, were conducted to observe the effect of the bacterial EPS extracted from Microbacterium sp. MAS133 isolated from a salt-affected soil on soil moisture release and aggregate stability at three (native, acidic and alkaline) pH values of clay fraction of a saline-sodic soil compared to a normal soil. Results showed that the processes and phenomenon of soil aggregate formation and stability in the colloidal form with interactive effect of biopolymers and SMA (suspended micro-aggregates) as well as water retention and release of a saline-sodic soil were all influenced by the bacterial EPS. Although extent and nature of the bacterial EPS-micro-aggregates interactions varied with pH, soil type and EPS concentration, the effect was consistent and persistent for extended time periods. Moreover, 16S rRNA gene sequence analysis showed that MAS133 belonged to Microbacterium hominis of the lineage of the Firmicutes and the EPS produced on sucrose medium were fructose biopolymers. Interaction of the bacterial biopolymer with soil constituents and a positive impact of the bacterial inoculation on soil aggregation around roots and mitigation of negative effects of salinity on plant growth observed in earlier studies suggest the EPS-producing bacteria a useful biological tool for reclamation of the salt-affected soils. Additionally, a strategy of provision of carbohydrate substrates to foster growth and production of the EPS by the bacterial populations living in the salt-affected soils through field water distributaries and the ‘bioretention’ or ‘biomonitoring’ cells could help overcome the economy and the environmental concerns associated with application and use of the bacterial inoculum in the field