Macromolecular and chemical features of the excreted extracellular polysaccharides in induced biological soil crusts of different ages

The development of biological soil crusts (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. 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 (i.e. dehydrogenase and sucrase activities) in induced biological soil crusts (IBSCs) of different ages displayed within an investigation area in Hobq Desert (Dalatequi County, Inner Mongolia, China). 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. In this study, the C-EPS and TB-EPS fractions extracted from the IBSCs of different age (4-, 6- and 8-years old IBSCs) were found present in comparable amounts but showed marked differences in terms of their molecular size distribution and monosaccharidic composition. C-EPS showed to be mostly constituted by sugar fractions with molecular weight (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. While the chemical and macromolecular characteristics of TB-EPSs did not show significant changes with the age of the crusts, the older IBSCs showed a lower content of low MW C-EPSs, as well a higher number of different types of monosaccharides constituting the C-EPS. Moving from these results, it can be hypothesized that C-EPSs, which are dispersed in the soil and thus more accessible, have been rather easily degraded by the heterotrophic microorganisms dwelling in mature IBSCs and reduced to low MW carbohydrates that are easily metabolized by chemoheterotrophs. This hypothesis is supported by the higher activity observed in older IBSCs of the two enzymes associated with sugar degradation in the soil, dehydrogenases and sucrases, that is consistent with an increased release of low MW carbohydrates in the crusts.The results obtained suggest that the colloidal fraction of the EPSs, which is more dispersed in the soil, is more easily degradable by the microflora, while the EPS fraction tightly bound to the soil particles, which is characterized by a high MW, plays a key role in giving a structural stability to the crusts and in affecting the hydrological behavior of the soil covered by IBSCs.