Microbes in dry soils lack food, not water

In fact, as shown by a theoretical study appeared in Soil Biology and Biochemistry, dry conditions inhibit the diffusion of soluble organic matter in the soil towards microbes, causing them to starve even if they can tolerate a drought. The corresponding author is Stefano Manzoni at the departments of Crop Production Ecology and Ecology.

Two strategies

The model describes how soil microbes respond to limited water – either by synthetizing osmolytes, chemical compounds that permit survival (drought tolerance), or by becoming dormant (drought avoidance). Microbes that invest large amounts of soil substrates in osmolytes remain active longer during a dry period, but this ‘expensive’ strategy does not always yield much in return, because in dry conditions diffusion limits the supply of carbon substrates. Therefore, microbes may be active but starving.
The alternative strategy is to invest little in osmolytes and thus lose the ability to remain active in dry conditions. These microbes therefore become dormant early during a dry spell. Surprisingly, this ‘cheap’ strategy allows in the long term a comparable substrate uptake, because it allows avoiding starvation and waiting for the next rain event to re-supply the microbial cells.

Ecosystem services

The article concludes by hypothesizing that the relative success of the different strategies of drought tolerance and avoidance depends on the history of previous rainfall events and the local climatology.
Because in many ecosystems dry periods are frequent, understanding microbial responses to changes in soil water is important to assess how soils store carbon and provide other ecosystem services.