Microbial community response to varying magnitudes of desiccation in soil: A test of the osmolyte accumulation hypothesis

Numerous studies have observed the physiological responses of soil microorganisms to water stress caused by soil drying, however, only a few have attempted to assess the microbial response in soil in situ. An experiment was conducted to analyze the change in extractable metabolites, particularly sugars and amino acids, in soil and the associated microbial community at various intensities of soil desiccation. Water potential was manipulated in two soils, Marietta and Sumter, representing relatively moist and drought-prone water regimes, respectively. The matric potential of the soils was maintained relatively moist at −0.03 MPa or lowered to −1.5, −4.5, −10, −20 and −40 MPa by air drying over ∼3 days. We hypothesized that microbial communities inhabiting the drought-prone Sumter would accumulate more osmolytes, and that the soil with a relatively moist water regime, the Marietta, may have communities less adaptable to water stress, have fewer osmolytes, and show evidence for greater microbial turnover and death. However, there was no evidence that the soils responded to drying by accumulating osmolytes or that there was greater microbial turnover and death related to soil type. Microbial community structure did change with drying, however, with greater fungal-to-bacterial biomass in the Sumter but not in Marietta soil. A significant increase of ∼10–25% in phenol sulfuric acid analyzable sugars (PSA-sugars) at intermediate levels (−4.5 MPa) of drying was observed compared to dryer and more moist conditions. However, the GC–MS derived quantities of polyols (glucitol, inositol and xylitol), sugars, and amino acids showed few strong and consistent patterns with level of desiccation. These results provide some of the first evidence that microbial communities in soil in situ do not strongly rely on these basic osmolytes to cope with typical soil water deficits. In natural soils, we propose that microbial communities respond differently to water deficits perhaps through re-allocation of C to cell wall mucilage, exopolysaccharides (EPS), and phospholipids, than organisms in culture, perhaps a consequence of low energy and limiting supplies of N.