Heterotrophic microorganisms are the agents that regulate decomposition of organic matter (OM). As such, soil microbial communities constitute one of the most influential controllers of the global carbon (C) cycle, dwarfing anthropogenic CO2 emissions. To achieve a mechanistic understanding of the global C cycle, we must thus identify and describe the factors that control microbial community processes in the environment, and how these feed-back to environmental change. I have assessed how actively growing microbial communities depend on one of the most influential environmental factors –temperature – by determining the community temperature relationship, including the cardinal points Tmin and Topt. I have also investigated how microbial communities respond to changes in the environmental temperature regime – warming – and how the microbial community temperature relationships depend on short-term changes, seasonal changes, and differences in mean annual temperatures between different ecosystems. Microbial endpoints considered include microbial growth rates, assessed with biomass production estimates for fungi and bacteria, along with respiration; a set of parameters that also enable estimates of the microbial C-use efficiency (CUE).
I show that soil microbial communities can adapt their temperature relationships to environmental changes. In laboratory conditions at warm temperatures, these changes can manifest within days, but shifts in temperature relationships to colder temperatures are substantially slower, remaining undetectable in laboratory microcosms even after exposure durations corresponding to a winter season in temperate ecosystems. When simulating soil warming in field experiments and studies, the laboratory results hold true: while experimental warming can shift microbial temperature relationships to become more warm-tolerant, adaptation to lower temperatures does not have time to manifest during seasonal changes in temperature in temperate ecosystems. These results suggest that it is rather heat-spells than cold-conditions that shape microbial temperature relationships in natural environments. Consistent with this interpretation, long-term differences in mean annual temperatures along continental climate gradients from Arctic to Mediterranean ecosystems, show a gradual and monotonic dependence of microbial temperature relationships on environmental temperatures; a relationship that is stronger with summer temperatures and weaker with winter temperatures. A microbial temperature relationship that is warm-adapted also consistently results in an increase in the temperature sensitivity of microbial process rates (i.e. higher Q10), suggesting a positive feedback in microbial process rates to warming. I also show that microbial communities with warm-adapted temperature relationships in field experiments optimise their CUE; suggesting that enhanced CUE is a trait selected for in natural environments.