Comparing soil microbial responses to drying-rewetting and freezing-thawing events

Climate change is expected to alter the frequency and intensity of soil drying-rewetting (D/RW) and freezing-thawing (F/TW) events, with consequences for the activities of microorganisms. Although both D/RW and F/TW events cause respiration pulses from soil to the atmosphere, it remains unknown whether the underlying microbial control is similar. Recent work has revealed that soil microbial responses to D/RW vary between two extremes: (Type 1) a resilient response, with a fast recovery of growth rates associated with a brief respiration pulse, or (Type 2) a sensitive response, where growth rates recover only after a lag period of no apparent growth associated with a prolonged respiration pulse. However, it remains unknown if these different microbial perturbation responses also occur after F/TW. Here, we directly compared microbial growth, respiration, and carbon-use efficiency (CUE) in response to D/RW and F/TW events. To do this, we selected two forest soils characterized by either sensitive or resilient responses to D/RW. We could confirm that D/RW induced either sensitive or resilient bacterial growth and respiration responses, but also that these distinct responses were found after F/TW. Additionally, F/TW resulted in shorter lag periods before the increase of bacterial growth, smaller respiration pulses, and lower levels of cumulative respiration, bacterial growth and fungal growth after the perturbation than did D/RW. These findings are consistent with a F/TW event imposing a similar stress on soil microorganisms to a D/RW event, but with lower severity. However, there was no significant difference in the microbial CUE between D/RW and F/TW, indicating that microorganisms maintain the stability of their C allocation in response to both types of perturbation. Altogether, our findings suggest that microbial communities are exposed to similar environmental pressures during D/RW and F/TW, implying that strategies to cope with drought can also provide protection to winter frost, and vice versa.