Physiological and Structural Response of Soil Microbial Communities and Their Metabolites to Varying Magnitudes of Osmotic and Matric Stress.

Numerous studies have undertaken the challenge to understand how soil microorganisms respond to various forms of water stress; however, there have been very few attempts to assess their physiological and functional responses in soil insitu. An experiment was conducted to study the physiological and structural responses of insitu soil microorganisms to the increasing levels of both osmotic and matric stress. Water potential was manipulated in aquentic (Marietta) and drought-prone (Sumter) soils. The soils were exposed to matric stress by air drying over several days to -1.5, -4.5, -10, -20 and -40 MPa and to similar osmotic water potentials by the gradual stepwise addition of NaCl to the soil. We hypothesized that amounts of sugars and amino acids would increase along the increasing water stress gradient and that the two soils would respond differently to water stress. The physiological response was measured by extracting the metabolites from the soil at different magnitudes of drying and analyzing them for sugars and amino acids (GC-MS) and structural changes was analyzed by extracting PLFAs. As hypothesized, sugar concentrations were greater in water stress-affected (~300µg/g of soil) compared to unstressed (~240µg/g of soil) soils. The concentration of polyols (glucitol, inositol and xylitol), in particular, increased by ~10- 30% as a result of water stress. Multivariate NMS analysis indicated that microbial communities changed both physiologically and structurally but that similar levels of water potential change caused greater change in salt compared to matric stressed soils. These results provide some of the first direct evidence that microbial communities in soil insitu do utilize sugars and alcohols to cope with water potential deficits.