Effects of freeze–thaw cycles on microarthropods and nutrient availability in a sub-Arctic soil

It is predicted that Arctic regions may experience an increase in mean temperature in the future. This will affect the frequency of severe climatic events such as summer droughts and freeze–thaw cycles. In order to understand the impact of recurring freezing and thawing on soil organisms and their environment, intact plant–soil samples from the sub-Arctic were subjected to a series of such events. Springtail and mite species composition and abundance were monitored at intervals throughout the experiment. Furthermore, nutrient content and mobilisation in the soil and soil microbial biomass and nutrient content were examined.There was no conclusive evidence that recurring freeze–thaw events had a negative effect on the investigated soil faunal groups, and the treatment even seemed to stimulate the abundance of Acaridida. Respiration of soil subjected to 16 freeze–thaw cycles was low when kept at −2 °C and high when kept at +2 °C, indicating rapid response of microbial activity even after long exposure to low and fluctuating temperatures. Oribatida and Gamasida displayed a higher abundance in controls kept at −2 °C for up to 80 days, compared to controls at +2 °C and the freeze–thaw treatment. The Collembola were unaffected by the temperature treatments, but increased in abundance over time. The microbial C:N ratio increased after 40 days at −2 °C, indicating a higher degree of fungal dominance and lower tolerance of bacteria to constant freezing, but not to freeze–thaw. The decline in inorganic and microbial P during the experiment, and the proportionally stronger decrease of inorganic and microbial P than N in frozen soil compared to +2 °C soil, suggests that P is affected more than N mineralisation by freezing.