MICROBIAL BIOMASS AND ACTIVITY IN SOIL AND LITTER UNDER PINUS-SYLVESTRIS, PICEA-ABIES AND BETULA-PENDULA AT ORIGINALLY SIMILAR FIELD AFFORESTATION SITES

Microbial biomass C and N, and activities related to C and N cycles, w ere compared in needle and leaf litter, and in the uppermost 10 cm of soil under the litter layer in Scots pine (Pinus sylvestris L.), Norwa y spruce (Picea abies L.) and silver birch (Betula pendula L.) stands, planted on originally similar field afforestation sites 23-24 years a go. The ground vegetation was differentiated under different tree spec ies, consisting of grasses and herbs under birch and pine, and mosses or no vegetation with a thick layer of needles under spruce. The C:N r atio of the soils was 13-21 and the soil pH(CaCl), 3.8-5.2. Both showe d little variation under different tree species. Microbial biomass C a nd N, C mineralization, net ammonification, net nitrification and N-2- fixing activity (acetylene reduction) did not differ significantly in soil under different tree species either. Birch leaf litter had a high er pH(CaCl), (5.9) than spruce and pine needle litter (pH 5.0 and 4.8, respectively). The C:N ratio of spruce needles was 30, and was consid erably higher in pine needles (69) and birch leaves (54). Birch leaves tended to have the highest microbial biomass C and C mineralization. Spruce needles appeared to have the highest microbial biomass N and ne t formation of mineral N, whereas formation of mineral N in pine needl es and birch leaves was negligible. Microbial biomass C and N were of the same order of magnitude in the soil and litter samples but C miner alization was tenfold higher in the litter samples.