Moisture as a regulator of microbial life in soil 

Abstract

<p><span>Higher frequency of drought periods as well as heavy precipitation events have been predicted by climate change models. These events strongly impact soil microbial communities, which act as gate-keepers for the carbon (C) exchange between soil and the atmosphere. Soil microorganisms can be directly affected by soil moisture, where drying will decrease microbial process rates. These processes can also be dramatically affected by fluctuations in moisture (i.e., drying and rewetting (DRW) events). When drought is followed by rain, a big CO2 release from soils to the atmosphere is observed. During these events respiration and microbial growth have been shown to be uncoupled. In addition, previous studies have observed that microbial growth and respiration have been shown to be uncoupled in two distinct ways. Either (i) microbes have a resilient response to rewetting, where growth rates begin to recover immediately after rewetting and increase linearly up to pre-disturbance levels, which coincides with maximal respiration rates immediately after rewetting, that then decreased exponentially, or (ii) microbes begin to recover their growth rates only after an extended lag period following which rates increase exponentially, coinciding with high and maintained rates of respiration, that increase further with growth. These responses result in relatively high or relatively low microbial carbon use efficiencies (i.e., the fraction of used C dedicated to growth) during the perturbation, respectively. Therefore, an understanding of how drought and DRW events influence soil microbial communities is essential to predict how terrestrial ecosystems will respond to climate change. </span></p><p>We aimed at gaining a deeper understanding of how soil microbial communities, and the processes they regulate are affected by drought and DRW events. Specifically, we wanted to understand (1) which are the factors that determine the two microbial responses to rewetting, (2) how the history of drought will determine microbial responses to drought and DRW and (3) whether bacteria and fungi respond differently to moisture and its fluctuations.</p><p>We found that (1) the “harshness” of drought perceived by microbes determined the type of microbial response to rewetting. Moreover, (2) the history of drought appeared to shape microbial responses to drought and DRW. We found that microbial communities from historically drier soils were more drought tolerant as well as more resilient with a higher carbon use efficiency upon rewetting. Finally, we showed that (3) fungi were generally more drought tolerant than bacteria, and that they could be equally or more resilient upon rewetting than bacteria.</p><p>In summary, to better predict how terrestrial ecosystems will respond to climate induced increase in drought periods and more intense rainfall events, ecosystem models should consider that bacteria and fungi are differently affected by moisture. In addition, the history of drought of the soils as well as the harshness of the disturbance should be taken into account to determine the responses of microbial communities to drought and DRW.</p><p> </p>