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Unstable snow cover and more frequent freeze-thaw events have been predicted for montane areas in southern Norway, where stable winters are common today. These systems are important contributors to the flux of carbon (C) and nitrogen (N) to air and water. Here we quantify and compare the effects of freeze-thaw on C and N release from soils collected below Calluna, Molinia or Sphagnum. Intact organic soil cores were subjected to four different freeze-thaw regimes for four consecutive 2-week periods:(1) slow cycling (SC) with one long freezing event during each 2-week period, (2) fast cycling (FC) with four short freezing events during each 2-week period, (3) permanent frost (PF) and (4) permanent thaw (PT). The freezing temperature was À5 1C and the thawing temperature was 5 1C. Before start of treatment, at the end of each 2-week period, and during postincubation periods, carbon dioxide (CO 2 ) emission as well as leachable dissolved organic C (DOC), dissolved organic N (DON), ammonium (NH 4 ), nitrate (NO 3 ) and absorbance at 254 nm were measured. In soils from all three vegetations, PF increased the release of CO 2 , DOC, DON and NH 4 compared with PT. SC caused some scattered effects whereas FC only resulted in some increase in NO 3 release below Molinia. Generally, the emission of CO 2 and leaching of DOC, DON and NH 4 increased in the following order: SphagnumoCallunaoMolinia. The release of NO 3 was greatest below Calluna. Our data suggest that vegetation cover and composition seem at least as important as increased soil frost for future winter fluxes of CO 2 , DOC, DON and dissolved inorganic N (DIN) from the soil to air and water. The freezing period needs to be sufficiently long to give significant effects.

We have manipulated the winter-time soil temperature regime of small headwater catchments in a montane heathland area of southern Norway to study the possible effects on concentrations and fluxes of inorganic nitrogen in runoff. The experiments included extra insulation of soils in two catchments to prevent subzero temperatures during winter, and removal of snow in two other catchments to promote soil frost. Increased soil temperatures during winter increased the springtime concentrations and fluxes of ammonium (NH4) and nitrate (NO3) in runoff. By contrast, snow removal with development of significant soil frost showed no systematic effects on mean concentrations or fluxes of inorganic N. The results from our experiments suggest that warmer soils during winter caused by exceptionally mild winters, or alternatively a heavy snowpack, imply a greater risk for inorganic N leaching in this region than a possible increase of soil frost events because of reduced snow cover.

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