Biological activity is known to occur in frozen soils, and the core of available literature is derived from studies on arctic, antarctic, and high elevation soils. Continued functioning of soil microorganisms during winter is now being evidenced in seasonally frozen agricultural and forest soils from temperate areas. The mechanisms involved in maintaining microbial activities in frozen and freeze-thawed soils are still under debate and point towards a new paradigm based on unfrozen water availability, solute movements, and nutrient recycling, with less influence of soil temperature. It has been reported that soil N transformations such as nitrification and mineralization may continue at detectable rates below the freezing point, whereas N immobilization would be more adversely affected by freezing. Soil clay content may also influence N transformations as rates tend to increase with clay content in frozen soils. Recent work demonstrated that microbial activities such as denitrification may even be stimulated by both freezing and thawing events, especially in fall-amended soils, and that freezing intensity during winter may induce higher N₂O fluxes during the spring-summer period. When monitored under field conditions, winter N₂O fluxes from seasonally frozen soils represented up to 80% of the total annual emissions.

Collecting and handling samples under freezing conditions is challenging, and existing methodologies must be adapted to the specific soil and environmental conditions. However, the growing evidence of continued and significant microbial activities during winter justifies putting more emphasis on this period to comprehend annual nutrient dynamics in temperate soils, and to better predict how soils may respond to and influence climate change. This symposium is aimed at bringing up-to-date knowledge of how and when biological processes take place under freezing, frozen, and thawing conditions in arctic, antarctic, and temperate soils.