Effects of freeze-thaw on C and N release from soils below different vegetation in a montane system: a laboratory experiment

Sammendrag

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 -5 degrees C and the thawing temperature was 5 degrees C. Before start of treatment, at the end of each 2-week period, and during postincubation periods, carbon dioxide (CO2) emission as well as leachable dissolved organic C (DOC), dissolved organic N (DON), ammonium (NH4), nitrate (NO3) and absorbance at 254 nm were measured. In soils from all three vegetations, PF increased the release of CO2, DOC, DON and NH4 compared with PT. SC caused some scattered effects whereas FC only resulted in some increase in NO3 release below Molinia. Generally, the emission of CO2 and leaching of DOC, DON and NH4 increased in the following order: Sphagnum < Calluna < Molinia. The release of NO3 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 CO2, 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.