Nitrogen transformations in soil under Pinus sylvestris, Picea abies and betula pendula at two forest sites

Microbial biomass N, net ammonification, net nitrification, denitrification potential, numbers of nitrifiers and the pH-dependency of nitrification were measured from the humus layer and mineral soil layers in adjacent stands of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and silver birch (Betula pendula L.). The trees had been established at two forest sites of different fertility approximately 60 years ago. The aim was to see whether the microbial and chemical characteristics of the soils differed under different tree species. The soil pH(H2O) varied from 3.8 to 5.0 and was lowest in spruce soil at both sites in all soil layers. Microbial biomass N, ammonification, nitrification and denitrification all differed in soils of pine, spruce and birch. The flush of N from fumigation varied from 36 to 67 micrograms N cm(-3) fresh soil in the humus layer, and from 13 to 50 microgram cm(-3) in the mineral soil layers. Denitrification potential with added nitrate was 2-29 ng N2O-N cm(-3) soil h(-1) in the humus layer and 0-28 ng in the mineral soil layers. Both tended to be lowest under spruce and highest under birch, at the fertile site in all soil layers and at the less fertile site in the humus layer. In the mineral soil layers of the fertile site and in the humus layer and upper mineral soil layer of the less fertile site the content of mineral N was highest under birch. Different populations of nitrifiers existed in the soils, regarding numbers, activity and pH-dependency. Only the nitrifier community in pine humus layer from the fertile site was adapted to acidic (pH 4.1) conditions. In an aerobic soil suspension the cumulative nitrate production of it was 32 micrograms cm(-3) soil in three weeks, compared to negligible production in the other soils. When the pH of the suspensions was raised to 6.0, all soils from the fertile site produced nitrate, but the production at the less fertile site was still negligible. Higher C-to-N ratios probably explained the low nitrification activity and numbers of nitrifiers at the less fertile site. Thus, there were differences in N transformations under pine, spruce and birch, but the changes depended also on the fertility of the site.