Lunch Seminar "The Biogeochemistry of Drought"

The world is a dry place: roughly 1/3 is arid or semi-arid, and drought is common even in mesic environments. Drought 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--the so-called "Birch Effect." However, increasingly, evidence suggests that this hypothesis is wrong. This raises two questions: 1. How, then do soil microbes deal with drought and yet remain viable? 2. What does cause the Birch Effect? An alternative explanation for the Birch Effect 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; and to develop approaches to scaling the physiological and chemical mechanisms up to the ecosystem scale.