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 we re 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. Microb ial biomass N, ammonification, nitrification and denitrification all differ ed in soils of pine, spruce and birch. The flush of N from fumigation varie d from 36 to 67 mu g N cm(-3) fresh soil in the humus layer, and from 13 to 50 mu g 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 les s fertile site in the humus layer. In the mineral soil layers of the fertil e site and in the humus layer and upper mineral soil layer of the less fert ile site the content of mineral N was highest under birch. Different popula tions of nitrifiers existed in the soils, regarding numbers, activity and p H-dependency. Only the nitrifier community in pine humus layer from the fer tile site was adapted to acidic (pH 4.1) conditions. In an aerobic soil sus pension the cumulative nitrate production of it was 32 mu g 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 p roduced nitrate, but the production at the less fertile site was still negl igible. Higher C-to-N ratios probably explained the low nitrification activ ity and numbers of nitrifiers at the less fertile site. Thus, there were di fferences in N transformations under pine, spruce and birch, but the change s depended also on the fertility of the site. (C) 1999 Elsevier Science Ltd . All rights reserved.