Comparing microbial biomass, denitrification enzyme activity, and numbers of nitrifiers in the rhizospheres of Pinus sylvestris, Picea abies and Betula pendula seedlings by microscale methods

Flushes of C and N from fumigation-extraction (FE-C and FE-N, respectively) , substrate-induced respiration (SIR), denitrification enzyme activity (DEA ) and numbers of NH4+ and NO2- oxidizers were studied in the rhizospheres o f Scots pine (Pinus sylvestris L.), Norway spruce [(Picea abies (L.) Karste n] and silver birch (Betula pendula Roth) seedlings growing in soil from a field afforestation site. The rhizosphere was defined as the soil adhering to the roots when they were carefully separated from the rest of the soil i n the pots, termed as "planted bulk soil". Soil in unplanted pots was used as control soil. All seedlings had been grown from seed and had been infect ed by the natural mycorrhizas of soil. Overall, roots of all tree species t ended to increase FE-CI FE-N, SIR and DEA compared to the unplanted soil, a nd the increase was higher in the rhizosphere than in the planted bulk soil . In the rhizospheres tree species did not differ in their effect on FE-C, FE-N and DEA, but SLR was lowest under spruce. In the planted bulk soils FE -C and SIR were lowest under spruce. The planted bulk soils differed probab ly because the roots of spruce did not extend as far in the pot as those of pine and birch. The numbers of both NH4+ and NO2- oxidizers, determined by the most probable number method, were either unaffected or decreased by ro ots, with the exception of the spruce rhizosphere, where numbers of both we re increased.