Dataset for: Heat wave-induced microbial thermal trait adaptation and its reversal in the Subarctic

Climate change predictions suggest that arctic and subarctic ecosystems will be particularly affected by rising temperatures and extreme weather events, including severe heat waves. Temperature is one of the most important environmental factors controlling and regulating microbial decomposition in soils; therefore, it is critical to understand its impact on soil microorganisms and their feedback to climate warming. We conducted a warming experiment in a subarctic birch forest in North Sweden to test the effects of summer heat waves on the thermal trait distributions that define the temperature dependencies for microbial growth and respiration. We also determined the microbial temperature dependences 10 and 12 months after the heat wave simulation had ended to investigate the persistence of the thermal trait shifts. As a result of warming, the bacterial growth temperature dependence shifted to become warm-adapted, with a similar trend for fungal growth. For respiration, there was no shift in the temperature dependence. The shifts in thermal traits were not accompanied by changes in α- or β-diversity of the microbial community. Warming increased the fungal-to-bacterial growth ratio by 33% and decreased the microbial carbon use efficiency (CUE) by 35%, and both these effects were caused by the reduction in moisture the warming treatments caused, while there was no evidence that substrate depletion had altered microbial processes. The warm-shifted bacterial thermal traits were partially restored within one winter but only fully recovered to match ambient conditions after one year. To conclude, summer heat waves in the Subarctic resulted in (i) shifts in microbial thermal trait distributions; (ii) lower microbial process rates caused by decreased moisture, not substrate depletion; and (iii) no detectable link between the microbial thermal trait shifts and community composition changes. 

Methods
In brief, soil and field characteristics: We measured gravimetric soil moisture (105°C for 24 h) and SOM content through loss on ignition (550°C for 12 h). Soil pH and electrical conductivity (EC) were determined in a 1:5 (w:v) soil:water extraction. Soil total carbon (TC) and total nitrogen (TN) were measured using Dumas dry combustion by a C/N 144 elemental analyzer (VarioMAX CN, Elementar, Germany). The normalized difference vegetation index (NDVI) as a proxy for plant productivity, the fraction of photosynthetically active radiation (fPAR), and the leaf area index (LAI; the projected area of leaves over the unit of measured ground area; m2 m-2) were determined with NDVI meter (SpectroSense2+, Skye, UK).

The temperature and moisture data were collected by using data loggers (TMS-4 29cm, TOMST®, Czech Republic) with 15 min resolution. The in situ soil (-8 cm), surface (0 cm), air (+15 cm) temperatures, and volumetric soil moisture (between 0 and -14 cm) were monitored.

Bacterial growth, fungal growth, and respiration were determined at ten different screening temperatures from 0 to 45°C in 5°C intervals. Bacterial growth was determined by radioactively labeled 3H-Leucine (Leu) incorporation into extracted bacteria from 0.5 g of fresh soils. Fungal growth was measured in 0.5 g of fresh soils using 14C-acetate (Ace)-in-ergosterol incorporation method. Microbial respiration was measured by a gas chromatograph equipped with a methanizer and flame ionization detector.