Abstract: The world is a dry place. Much of it is already arid or semi-arid, and drought is expected to become more frequent and intense. The effects of drought on biogeochemical processes are profound, but at times counter-intuitive. Current models assume that as soils dry, biogeochemical processes merely slow down and microbial populations decline; when soils rewet, processes just pick up again. All these assumptions are wrong. Rewetting a dry soil typically causes a huge burst of respiration, although it remains unclear where the carbon comes from—microbial C vs. C released from mineral surfaces or aggregrates. But surprisingly, when soils become extremely dry, the biomass of microorganisms in soil increases. Pools of extractable C and N increase. But, even very slight increases in moisture can switch biogeochemical patterns back to the “moist” conditions: biomass and pools of available resources decline. The causes of some of these dynamics remain obscure, but appear to result from the interactions of microbial stress physiology and the extent of hydraulic connectivity at the microscale in the soil.

About the Author: Josh Schimel did his Ph.D. in Soil Microbiology at UC Berkeley in 1987. After postdocs in Aberdeen, Scotland, and East Lansing, Michigan, he became an assistant professor at the University of Alaska, Fairbanks. He moved to UC Santa Barbara in 1995, where he is now a professor and Chair of the Environmental Studies Program. His research focuses on soil C and N cycles in Arctic and Mediterranean ecosystems: areas dominated by a stressful environment.

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