The role of the exopolysaccharides in enhancing hydraulic conductivity of biological soil crusts

Biological soil crusts (BSCs) are highly specialized topsoil microbial communities commonly found in arid and semiarid environments, permeated by a polymeric matrix of polysaccharides. BSCs can in principle influence edaphic properties such as texture, pore formation and water retention, which in turn determine water distribution and biological activity in dry lands. This paper investigates the influence of biotic and abiotic factors on BSC hydraulic conductivity, a parameter gauging the ease with which water can move through the pore spaces. Texture, phototroph abundance, microbial composition, and extracellular carbohydrate content were considered as potentially relevant parameters in a correlational study of BSC samples that spanned 1.5 orders of magnitude in hydraulic conductivity. A newly developed, non-destructive extraction method enabled us to directly quantify the specific role of extracellular polysaccharides on soil permeability on a variety of samples. Hydraulic conductivity showed a strongest correlation with texture (positive with sand content, negative with silt and clay). A weaker negative correlation with carbohydrate content, especially with polysaccharides having a molecular weight < 100 kDa, was also detected. In multiple regression analyses texture (silt content) was sufficient to explain most of the variation in hydraulic conductivity However, experimental removal of polymeric carbohydrates, resulted invariably in a substantial decrease in hydraulic conductivity for any given sample (between 1.7 and 3.3 fold). Our results suggest that while soil texture determines overall hydraulic conductivity in BSCs, the presence of exopolysaccharides can significantly enhance it, likely by conferring a spongy structure to a BSC thus increasing the number of waterways within it.