Resource acquisition is more sensitive than carbon storage in soil microorganisms under climate extremes

Drought and warming can disrupt soil microbial processes and ecosystem functioning. Although soil microorganisms can exhibit physiological adjustments to drought, it remains unclear how they allocate resources between extracellular resource acquisition, potential oxidative metabolism, and carbon storage during drought and recovery, particularly under constant warming and/or heat waves.

Here, we tested the effects of drought on microbial resource allocation strategies across warming regimes during the resistance and recovery phases. We performed a full-factorial outdoor mesocosm experiment combining drought with constant warming and periodic heat waves, applied individually and in combination. We measured the potential activities of extracellular enzymes as proxy for the acquisition of microbial resources, the activity of dehydrogenase as a proxy for the potential active oxidative metabolism, and microbial glycogen pools as a proxy for carbon storage. We also quantified drought legacy effects by measuring microbial functioning before the new drought treatments, capturing the influence of the drought imposed in the previous year.

During the resistance phase, dehydrogenase activity and glycogen pools remained stable, despite reduced extracellular enzyme production, while enzyme allocation shifted towards oxidative enzymes associated with acquisition of recalcitrant C in warming regimes. One month after rewetting, all microbial proxies no longer differed from the control soil moisture conditions. Drought legacy effects were observed in extracellular enzymes, dehydrogenase activity, and glycogen pools, with glycogen exhibiting the strongest legacy effect.

We conclude that the asymmetrical responses of extracellular resource acquisition and internal C storage to drought and warming may function as strategies for microbial survival in increasingly variable climates.