Poster, ISOECOL 6, Honolulu, Hawaii: 25.08.2008 - 29.08.2008
Abstract:
Nitrous oxide (N2O) is a potential and long-lasting greenhouse gas of the atmosphere. The atmospheric N2O concentration currently increases by about 0.3 % per year and reached 319 ppbv in 2005. With a contribution of about 70 % soils act as the main source for atmospheric N2O. Soil N2O emissions originate from microbial nitrification and denitrification, with the latter process also potentially being able to consume N2O. Both processes are mainly driven by soil temperature, soil moisture and substrate availability. Changes in meteorological conditions as predicted for the future are, therefore, expected to affect N2O emissions from soils. In this study we investigated effects of experimentally induced drying/rewetting and freeze/thaw events on soil N2O emissions in a mature Norway spruce forest in the Fichtelgebirge (NE Bavaria, Germany). Drought was induced by roof constructions and freezing by snow removal. The experiments were run in three replicates each. Unmanipulated plots served as controls. In addition to N2O flux measurements between soil and atmosphere we analysed N2O concentrations and stable isotope signatures (δ15NN2O and δ18ON2O) in soil air collected along soil profiles. The latter approach provides information on localisation of N2O sources and sinks as well as identification of N2O production and consumption processes. Drought reduced the N2O emission from the soil or even turned the forest soil temporarily to an N2O sink. N2O emission peaks after rewetting could not compensate for the drought effect. Soil frost caused a burst of N2O emission. Soil air N2O concentration and stable isotope profiles provide a new and hitherto almost unconsidered mechanistic explanation for all of these observations. N2O concentration in the soil air decreased in most cases exponentially from the subsoil to the soil surface. This observation identifies microbial activity in the subsoil as source for N2O and diffusion to the soil surface along a concentration gradient. A shift in the N2O isotope signature along the concentration gradient towards increasingly positive δ values indicates, furthermore, a simultaneous microbial N2O consumption (reduction to N2). Drought reduced the source strength of the subsoil for N2O while simultaneously the sink strength of the topsoil for N2O remained constant. Both of these factors resulted in the temporary occurrence of below-atmospheric N2O concentrations in the air of the topsoil and thus a soil sink function for atmospheric N2O. Frost in the topsoil was the only exception for these trends in N2O concentration and isotope signature. Under conditions of soil frost the topsoil served no longer as a sink for N2O, thus leading to the observed burst in N2O emission. The until now almost unconsidered sink function of soils for N2O sheds a new light on the hitherto poorly resolved global N2O budget. We suggest the consideration of a soil N2O sink function for future model calculations on global N2O budgets.