Microorganisms, molecules, and moisture: Drought induced soil community dynamics and their impact on ecosystem function

Background/Question/Methods The re-wetting of dry soil is known to cause small but relatively important changes in microbial community structure. Additionally, large pulses of C and N occur following the re-wetting of dry soil, and are considered key contributors to the overall cycling of these two elements. Yet, the operational linkages that occur between shifts in the physiology and structure of microbial communities and the dynamics of C and N following the re-wetting of dry soil are not fully explored. One example of the post wetup process that remains unproven is the contribution of microorganisms to the flush of organics following the rewetting of dry soil and whether this flush is composed of microbial derived osmolytes. Results of the molecular composition of microbial cells and the soluble carbon pool will be presented with the objective of 1) understanding the microbial adaptation and response to water stress, and 2) its implications to ecosystem C and N flows. One-dimensional and two-dimensional nuclear magnetic resonance (NMR) spectroscopy were conducted in combination with GC-MS on solutions of 0.01 M K₂SO₄ with and without chloroform, after and before rewetting of dry soil, respectively. The organics dissolved in the K₂SO₄ solution with and without chloroform represented intracellular and extracellular microbial pools, respectively.

Results/Conclusions Several types of sugars and alcohols (glucose, glucitol, glycerol, ribitol, xylitol, mannitol) were detected in the microbial extracts, and the concentrations of a number of these sugars and alcohols, putative osmolytes, increased in concentration by 1.5 to 3X during soil drying. None of these putative osmolytes, however, were found in the pool of soluble organics that can be extracted directly following the re-wetting of dry soil. Hence, osmolytes appear to be important components of the microbial stress response to soil drying and their mineralization could be an important part of the carbon-dioxide flush that occurs, but it is still unknown what molecules contribute to the increase in soluble carbon following the re-wetting of dry soil. The dynamics and general chemical nature of this soluble carbon pool will be presented.