Repeated freeze-thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil

Freezing and thawing may substantially influence the rates of C and N cycling in soils, and soil frost was proposed to induce NO3- losses with seepage from forest ecosystems. Here, we test the hypothesis that freezing and thawing triggers N and dissolved organic matter (DOM) release from a forest soil after thawing and that low freezing temperatures enhance the effect. Undisturbed soil columns were taken from a soil at a Norway spruce site either comprising only O horizons or O horizons + mineral soil horizons. The columns were subjected to three cycles of freezing and thawing at temperatures of -3 degrees C, -8 degrees C, and -13 degrees C. The control columns were kept at constant +5 degrees C. Following the frost events, the columns were irrigated for 20 d at a rate of 4 mm d(-1). Percolates were analyzed for total N, mineral N, and dissolved organic carbon (DOC). The total amount of mineral N extracted from the O horizons in the control amounted to 8.6 g N m(-2) during the experimental period of 170 d. Frost reduced the amount of mineral N leached from the soil columns with -8 degrees C and -13 degrees C being most effective. In these treatments, only 3.1 and 4.0 g N m(-2) were extracted from the O horizons. Net nitrification was more negatively affected than net ammonification. Severe soil frost increased the release of DOC from the O horizons, but the effect was only observed in the first freeze-thaw cycle. We found no evidence for lysis of microorganisms after soil frost. Our experiment did not confirm the hypothesis that soil frost increases N mineralization after thawing. The total amount of additionally released DOC was rather low in relation to the expected annual fluxes.