Prolonged frost increases release of C and N from a montane heathland soil in southern Norway

Montane heathlands of southern Norway are important contributors of carbon (C) and nitrogen (N) to aquatic and marine systems. Climate change may give milder winters and decreasing snow cover, producing pronounced soil temperature variations, including freeze-thaw events. A controlled experiment in the laboratory was set up to investigate the effects of freeze-thaw on C and N release. Undisturbed soil columns were subjected to different temperature regimes, including freeze-thaw cycles and one permanent frost program. Leachates after permanent frost had a significant increase in DOC, DON and NH4-N concentrations. Slow temperature cycling with soil temperature periodically well below 0 had a similar, but far less pronounced effect on leachate composition. On the other hand, the effect of fast temperature cycling, with soil temperature periodically slightly below 0, was not significantly different from the permanent thaw treatment. Permanent frost, and to some extent slow cycling, caused a significant decrease in NO3-N concentration, and generally gave increased levels of organic relative to inorganic C and N species. Our data suggest that at realistic temperatures, only prolonged soil frost induces pronounced effects on C and N release.