Microbial activity in frozen soils has recently gained increasing awareness and the fact that soil microorganisms can perform significant metabolic activity also under such conditions is apparent. However, to what extent microbial activity is constrained by the environmental conditions prevailing in a frozen matrix is still very uncertain. For example, observed temperature responses (e.g. CO₂ production rates expressed as activation energies (Ea) or Q₁₀-values) across, and below, the freezing point appears very different as compared to the responses above the freezing point. In addition, freezing of the soil water drastically reduces the amount of liquid water in the soil and, concomitantly, the water potential of the remaining liquid water is strongly reduced. Apart from directly influencing water availability per se, soil freezing also affects processes related to substrate transport through the soil matrix because e.g. connectivity between pores harboring active microorganisms becomes more restricted. These are just a few examples of the conditions microorganisms have to adapt to if they want to remain active when soils freeze. This presentation will address how the fundamental environmental factors of temperature, liquid water availability and substrate availability combine to regulate rates of catabolic and anabolic microbial processes in frozen soils. The presented results are gained from investigations of the surface layers of boreal forest soils that seasonally are subjected to freezing. Some of the main results are:

The amount of liquid water is an integral factor regulating rates of microbial activity in the frozen soil matrix and can also explain the unexpected deviations in observed temperature responses of biogenic CO₂ production in frozen soils. The capacity for a specific soil to retain liquid water at sub-zero temperatures can be related to the structural composition of the soil organic matter. The potential microbial partitioning of carbon substrates to catabolic and anabolic processes are similar both above and below the freezing point, showing that active microbial growth may proceed even if soils are frozen.

To conclude, the soil microorganisms in the investigated soils appear very adapted to seasonal freezing with respect to their metabolic performance. However, the metabolic response to controlling factors may change substantially across the freezing point, and also the patterns of interaction among controlling factors are affected.