Microbial temperature dependences in soil: The belowground feedback to climate change

Sammanfattning: Since the Industrial Revolution, human activities have elevated atmospheric CO² concentrations. The consequences of this include rising temperatures, shifts in precipitation patterns, and increased intensity and frequency of extreme weather events, such as heat waves and droughts. Elevated temperatures can accelerate microbial activity in soil, potentially resulting in an increased rate of soil organic matter (SOM) decomposition. This increased microbial decomposition may, in turn, lead to a release of CO², contributing to a positive feedback loop amplifying climate warming. To understand the microbial feedback to warming, I studied the processes leading to carbon (C) accumulation through microbial growth and CO² release via microbial respiration. I determined the temperature dependence of microbial growth and respiration to assess how these process rates change with altered temperatures. The results of this thesis indicate that (i) the microbial temperature dependence is not dependent on soil moisture. This validation through an empirical test is important, as most ecosystem models employ a distinct temperature dependence that operates independently of soil moisture. In addition, (ii) the temperature dependence of bacterial growth can become warm-shifted within one growing season due to a summer heat wave simulation in the field and with a similar trend for fungal growth. The warm-shifted bacterial growth temperature dependence fully recovered within a year and matched the temperature dependence at ambient conditions. These findings highlight the fast microbial responses to a heat wave and the long-lasting legacy of such extreme weather events. The results also indicate that (iii) the microbial temperature dependence varies systematically with environmental temperatures along a wide climate gradient in Europe. Microbial communities showed warm-shifted temperature dependences in warmer ecosystems and cold-shifted temperature dependences in colder areas. Finally, (iv) empirically determined microbial temperature dependences were incorporated into a dynamic vegetation model LPJ-GUESS. Specifically, separate temperature dependence for microbial growth and respiration were employed to represent C sequestration and emissions from soils in response to temperature variations. In addition, the microbial temperature dependences were allowed to adjust to the climate that they encounter. Therefore, the microbial thermal traits can become climate-specific and adjust to changes in thermal regimes.