Nitrification in organic and mineral soil layers in coniferous forests in response to acidity

In order to study the response of nitrification to changes in pH in the FH layer and the upper B horizon, sifted samples from three spruce sites in SW Sweden were treated with CaCO3, H2SO4, urea or nitrifying humus in the laboratory and incubated for 110–180 days. Acetylene was used to distinguish between autotrophic and heterotrophic nitrification. In the FH layer, net nitrification was stimulated by CaCO3 and tended to be reduced by H2SO4. In samples where nitrate production was not increased by CaCO3, inoculation with nitrifying humus had a stimulating effect. In the B horizon, there was always a net nitrate production, but addition of CaCO3 or urea had practically no effect. H2SO4 reduced net nitrification also in the B horizon. Acetylene inhibited most of the nitrification in both soil layers. The results showed that at these sites nitrification was more pH-dependent in the FH layer than in the B horizon. The most acid FH layer seemed to have small or inactive populations of nitrifiers that responded to increased pH only after several years. This shows that generalisations about nitrification in forest soils cannot be made from studies of the upper soil layers alone.