Will experimental warming change soil microbial temperature thresholds for heat-tolerance?
Authors: Cruz Paredes Carla , Tajmel Daniel , Rousk Johannes 
Published: March 2022

&lt;p&gt;Temperature is one of the most important environmental factors controlling both microbial growth and respiration. However, there are multiple gaps in our understanding of microbial responses to warming. Soil microbial temperature relationships correlate with the climate, and microorganisms in field experiments become more warm-adapted in response to chronic warming. It has been suggested that it is the warmesttemperatures of the year that characterize microbial temperature relationships. Therefore, the intensity of summer is likely to shape the soil-atmosphere C exchange. It is important to note that with climate change the frequency of extreme events including summer heat spells will increase. Therefore, it is important to know the resilience of microbial communities to seasonal heat stress, and to find the temperature threshold above which microbial communities can be compromised by direct temperature effects.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;In this study we sampled soils from one temperate site (MAT=8.7&amp;#176;C) and one subarctic site (MAT=-3.1&amp;#176;C). Each site had been exposed to replicated warming treatments during one summer season (+ 5&amp;#176;C) to simulate summer heat waves. After field treatments the different soils were exposed to new temperature regimes during one month controlled laboratory incubations: &amp;#160;26, 32, 38 and 44&amp;#176;C to simulate extreme and chronic summer heat spells and to find microbial temperature thresholds. Bacterial growth temperature relationships were evaluated at 5, 11, 19 and 28 days after incubation and fungal growth and respiration rate temperature relationships were evaluated at the end of the incubations. We used the square root (or &amp;#8220;Ratkowsky&amp;#8221;) model to represent microbial temperature relationships and to determine the microbial community&amp;#8217;s temperature adaptation. This model enables the estimation of cardinal points, that describe the temperature relationship, T&lt;sub&gt;min&lt;/sub&gt;, T&lt;sub&gt;opt&lt;/sub&gt;, and T&lt;sub&gt;max&lt;/sub&gt;. Any of these can be used to screen for changes in the temperature relationships, where higher values denote warm-adapted relationships, and vice versa.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The temperate site showed higher T&lt;sub&gt;min &lt;/sub&gt;(&amp;#916;T&lt;sub&gt;min&lt;/sub&gt;&amp;#8776;2&amp;#176;C), T&lt;sub&gt;opt &lt;/sub&gt;(&amp;#916;T&lt;sub&gt;opt&lt;/sub&gt; &amp;#8776;1&amp;#176;C), and T&lt;sub&gt;max&lt;/sub&gt; (&amp;#916;T&lt;sub&gt;max&lt;/sub&gt; &amp;#8776;1&amp;#176;C) values compared to the subarctic site. Additionally, we found that warming treatments in both sites had warm-adapted temperature relationships with higher T&lt;sub&gt;min&lt;/sub&gt; (&amp;#916;&amp;#8776;2&amp;#176;C), T&lt;sub&gt;opt &lt;/sub&gt;(&amp;#916;&amp;#8776;1&amp;#176;C), and T&lt;sub&gt;max&lt;/sub&gt; (&amp;#916;&amp;#8776;1&amp;#176;C) values than control treatments. We found that the sites responded differently to the laboratory heat wave simulation treatments. In the temperate site we found that temperature relationships shifted to warm-adapted when incubated at 38&amp;#176;C (T&lt;sub&gt;min&lt;/sub&gt; +2&amp;#176;C) and 44&amp;#176;C (T&lt;sub&gt;min&lt;/sub&gt; +3&amp;#176;C) while the subarctic site shifted already at 32&amp;#176;C (T&lt;sub&gt;min&lt;/sub&gt; +3&amp;#176;C). These shifts were gradual for 32&amp;#176;C (28 days), while for 38&amp;#176;C and 44&amp;#176;C they occurred after 11 and 5 days, respectively. One summer season of field warming treatments (+5&amp;#176;C) did not systematically affect the responses to the heat wave simulation. Our results show that temperature relationships remained unaffected with incubation temperatures up to the soil community&amp;#8217;s T&lt;sub&gt;opt &lt;/sub&gt;for growth. Exposure to temperatures matching or exceeding the T&lt;sub&gt;opt&lt;/sub&gt; caused a shift in the temperature relationships towards higher temperatures. Therefore, summer heat spells could affect temperature relationships when the T&lt;sub&gt;opt &lt;/sub&gt;is exceeded, which could lead to changes in bacterial community activity, and be likely associated with significant alterations in ecosystem functioning.&lt;/p&gt;