Soil Ecology in a Dry World.

Soil microbiologists have long focused on carbon and nitrogen as the essential resources regulating microbial function in soil, ultimately water is the critical resource. It is essential for cellular function, is the essential solvent, and is the primary transport medium for solutes and microbial predators. Despite the complex ways in which water affects soil microbes, we have generally collapsed them to the simple pattern: microbial activity increases with moisture to an optimum, beyond which soils become anaerobic and activity declines. Physiological theory has also argued that as soils dry, micoorganisms must accumulate solutes to remain hydrated—when soils are rewet, these osmolytes would have to be released instantly —either through controlled physiological mechanisms or when cells rupture as water floods into them; either mechanism could explain the respiration pulse observed on rewetting (the “Birch Effect”). Yet, “bacteria don’t read textbooks”: processes occurring when soils are dry and are then rewet can be surprising and counterintuitive. Microbial biomass can increase under drought, microbes may not accumulate osmolytes under drought, and rewetting may do more to fuel microbial activity than to stress cells. This talk will explore work we have been doing to develop a new perspective on how soil moisture regulates microbes and soil biogeochemistry under drought and rewetting and how they regulate the fate of carbon in soil.