Print PageDrought is stressful for soil microorganisms. Reduced water potentials pose a physiological stress, while reduced diffusion limits resource availability at a time when microbes may need resources to pay the costs of surviving stress. Theory and culture studies have suggested that under water stress, microorganisms accumulate organic compounds as osmolytes, reducing cellular water potentials to keep cells hydrated. Upon rewetting, those compounds have been thought of as threats—if microbes cannot dispose of them, water would flood in and lyse the cells, providing a flush of resources for surviving microbes to grow on. This has been thought to be responsible for the pulse of respiration on rewetting. An alternative mechanism, however, is mobilization of C by physical and chemical processes associated with rewetting. We used a combination of lab studies of process and population dynamics associated with drying and rewetting coupled to modeling studies to explore these mechanisms and dynamics.
Results/Conclusions
Physical mobilization of SOM appears more important than the release of physiological materials through multiple dry/wet cycles:
1. Laboratory studies show that multiple cycles mobilize more C than was initially present in the microbial biomass.
2. In deep soils from a California grassland, the C released comes from a pool with a turnover time of 600-800 years, suggesting it did not come from a pool of freshly synthesized materials.
3. Microbes in this soil do not appear to synthesize simple organic compounds as inducible osmolytes during the long dry summer.
We have used two approaches to incorporate these effects in biogeochemical models of arid environments. One is a simple empirical modification to the DAYCENT model while the other is a mechanistic model that incorporates microbial dry-season processes.
This work shows that the biological and physico-chemical processes associated with drought and dry/wet cycles alter C and N dynamics relative to those that would be predicted based on either pure-culture microbiology or biogeochemical studies on moist soils.
