A comparative analysis of soil microbial responses to freezing-thawing and drying-rewetting
Authors: Rousk Johannes , Li Jintao , Brangarí Albert , Hicks Lettice 
Published: March 2022

&lt;p&gt;Climate change will lead to more pronounced and frequent drying-rewetting (DRW) and freezing-thawing (FTW) events. It is well known that DRW will induce powerful dynamics in microbial and biogeochemical process rates, and that a rewetting event may trigger two distinct bacterial growth responses depending on the severity of the drought stress, and the microbial drought tolerance. Bacteria either start growing immediately and linearly following rewetting (Type 1), associated with exponentially decreasing respiration from maximal levels immediately after rewetting, or bacterial growth starts increasing exponentially after an extensive lag period (&lt;30h) after rewetting, co-occurring with maintained high levels of respiration during the lag, and a subsequent secondary increase in sync with the growth increase (Type 2). Events of FTW present mechanistic similarities to those of DRW since soil water availability is reduced by both drying and freezing, while both rewetting and thawing can reverse the effects. Despite these similarities, it remains unknown if they share a similar microbial regulation.&lt;/p&gt;&lt;p&gt;We hypothesized (H1) that type 1 and 2 microbial responses to DRW would also be induced by a FTW cycle, but with less pronounced differences than between DRW, due to the smaller water potential reduction by freezing than drying. We also hypothesized (H2) that repeated DRW cycles would gradually induce faster growth recoveries to DRW, but also to FTW, and vice versa. Finally, we hypothesized (H3) that a higher frequency of FTW cycles in a field-experiment would give rise to enhanced microbial resilience to FTW cycles. We addressed these hypotheses by (1) comparing microbial responses to DRW and FTW in one type 1 soil, and in one type 2 soil. We then (2) exposed the set of two soils to a regime of 1-3 FTW or DRW cycles, followed by another characterization of the microbial responses to DRW and FTW. Finally, (3) we assessed if exposing a subarctic soil to winter-warming field treatments (+5&amp;#176;C), increasing the number of FTW events, affected the microbial responses to a FTW cycle.&lt;/p&gt;&lt;p&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160; We found that (H1) microbes recovered growth rates more slowly and respired more in response to DRW than to FTW in both type 1 and type 2 soils, and fungal growth was promoted during the DRW induced lag period of bacteria. We also found that (H2) both an increasing number of cycles of FTW and DRW could accelerate the bacterial growth recovery to both FTW and DRW while fungal growth recovery was unaffected, but that exposure to 1-3 DRW cycles accelerated bacterial growth responses to both FTW and DRW more than exposure to 1-3 FTW cycles could, in both type 1 and 2 soils. Finally, (H3) winter-warming induced an accelerated bacterial growth recovery, slowed the fungal growth recovery, and marginally reduced the respiratory pulse induced by one experimental FTW event in the laboratory. &amp;#160;&amp;#160;&lt;/p&gt;