N2O emission in a Norway spruce forest due to soil frost: concentration and isotope profiles shed a new light on an old story
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In mountain regions of Central Europe an increase of soil frost periods is predicted for this century due to reduced snow fall. To investigate the effects of freezing and thawing on soil N2O fluxes in a mature Norway spruce forest in the mountainous Fichtelgebirge, Germany, the natural snow cover on three experimental plots was removed to induce soil frost. Three plots with natural snow cover served as controls. Soil N2O fluxes were recorded in biweekly to monthly intervals during the frost and subsequent thawing period of the below-average cold winter in 2005/2006 and in the above-average warm winter in 2006/2007. In addition, N2O concentrations and isotope signatures in soil air were measured along soil profiles in six different depths (from 6 to 70 cm). The soil of the snow removal plots was frozen down to 15 cm depth from January to April 2006 while the soil of control plots remained unfrozen under snow cover. Both soil freezing and thawing resulted in almost tenfold enhanced N2O fluxes on snow removal plots contributing 84% to annual N2O emissions. In the subsequent winter without soil frost no effects were observed. Vertical gradients of N2O concentrations together with isotope abundance suggest that the subsoil of all plots was a probably weak, but continuous N2O source throughout the year. Isotope signatures and N2O concentration gradients in the soil profile indicate that microbial N2O production and reduction of N2O to N2 did not or just marginally occur in frozen soil layers of the snow removal plots. Consequently, elevated N2O fluxes in the late winter were attributed to the release of accumulated N2O originating from the subsoil. At unfrozen soil, however, N2O emissions were reduced due to a shift of the N2O production-consumption ratio towards more consumption in the topsoil of both the control and snow removal plots. These findings contradict the general assumption that N2O production in the organic layer is responsible for bursts of N2O due to soil frost.