Microbial secreted exopolysaccharides of induced biological soil crusts in Inner Mongolian desert soils: chemical characteristics and role in affecting water-soil relations

In nature, Biological Soil Crusts (BSCs) are highly specialized communities constituted by cyanobacteria, green algae, fungi, mosses, lichens and heterotrophic bacteria. They inhabit the first few millimeters of the soil surface in arid and semiarid environments where, due to harsh environmental conditions, the growth of most of the other organisms is restricted. The development of BSCs is widely recognized as beneficial to soil fertility due to their contribution to the stabilization of soils and to the increase in their carbon and moisture content. The induction of BSCs through spray-inoculation of sandy soils with suitable cyanobacteria was shown to be a beneficial and exploitable tool to trigger soil rehabilitation and counteract desertification in a number of experimental sites in Inner Mongolian deserts. An important role in these processes is played by the extracellular polysaccharidic (EPS) matrix embedding microbial cells and soil particles in BSCs. The present study was aimed at investigating the molecular and chemical features of the EPSs and the degradation processes of the polysaccharidic matrix in induced BSCs (IBSCs) of different ages displayed within an investigation area in Hobq Desert (Inner Mongolia, China). The influence of the EPSs in affecting the hydrological behaviour of IBSCs as well as in capturing and maintaining moisture was also investigated. Two operationally-defined EPS fractions, the colloidal (C-EPS) and the EDTA extractable (tightly bound, TB-EPS) fractions, were analyzed. In BSCs, C-EPSs are loosely bound to cells and sediments while TB-EPSs are tightly bound to the crustal biotic and abiotic constituents of the crusts. C-EPS showed to be mostly constituted by sugar fractions with MW distributed in the range 2000 - 76 kDa and in the range 64 - 0.34 kDa. Conversely, the TB-EPSs showed to be prominently constituted by one fraction having a MW in the range 2000 - 76 kDa. The presence of EPSs in IBSCs also induced a significant decrease in the hydraulic conductivity of IBSCs in comparison with bare sandy soil and contributed to their capability of trapping and retaining humidity. The results obtained suggest that C-EPSs, which are dispersed in the soil, are more easily degradable by the heterotrophic microflora, while the TB-EPSs, which are characterized by a high MW, play a key role in giving a structural stability to the crusts and in affecting their hydrological behavior.