N2O and CH4-fluxes from soils of a N-limited and N-fertilized spruce forest ecosystem of the temperate zone

Based on a 3-year data set from measurements of N2O fluxes from soil of a N -limited spruce forest ecosystem it could be demonstrated for the first tim e that such soils can function as a sink rather than a source for atmospher ic N2O. The results suggest that N2O uptake from the atmosphere into the so il is catalyzed by soil denitrifiers which are able to use N2O from the atm osphere instead of nitrate as an electron acceptor for denitrification due to severe limitations of nitrate in the soil. This interpretation is in acc ordance with the finding that net nitrate production via nitrification was zero or only marginal in the soil of the unfertilized site. On the other ha nd, the results strongly indicate that atmospheric N-input - simulated in t his experiment by ammonium sulfate application (150 kg N ha(-1)) to the for est soil - can lead to a change of the function of the soil of a N-limited forest ecosystem from a sink to a net source of atmospheric N2O. This chang e was most likely due to stimulation of N2O production via nitrification an d denitrification after N-fertilization. N-fertilization lead to a partial inhibition of atmospheric CH4 oxidation. However, this inhibition lasted on ly short term after N-fertilization and even changed at the end of the obse rvation period to a weak stimulation of CH4 uptake activity at the N-fertil ized site when soil ammonium concentrations at this site had decreased to v alues which were only slightly higher as compared to the unfertilized site. This indicates that at the unfertilized site atmospheric methane oxidizers were N-limited for growth.