Mycorrhizas and C and N transformations in the rhizospheres of Pinus sylvestris, Picea abies and Betula pendula seedlings

Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.) and silver birch (Betula pendula L.) seedlings were grown in a greenhouse for four mo nths in three different soils. The soils were from a field afforestation si te on former agricultural land: soil from a pine site, soil from a spruce s ite and soil from a birch site. Pots without seedlings were included. The a im was to discover, independent of the effects of the different quality of aboveground litter and microclimate under the tree species, whether the roo ts change the microbial activities and chemical characteristics of the soil , whether the changes are dependent on the tree species, and whether the ch anges vary in different soils. Pine, spruce and birch had, on average, five, one and six meters of roots, respectively. Birch had by far the highest number of short root tips, on av erage 11 450 per seedling, compared to 1900 and 450 in pine and spruce seed lings, respectively. The majority of the short roots of pine and spruce wer e brown sheathed mycorrhizas, and those of birch were mycorrhizas in an ear ly stage of development. The seedlings caused no major changes in either the soil pH or the concentr ations of nutrients in the soils, but did affect the microbial characterist ics of the soils. The effect of the tree species did not differ in differen t soils. Microbial biomass C and N, C mineralization rate and the concentra tion of ergosterol were all higher under birch and pine than under spruce a nd in plantless soils. Nitrate concentrations were lowest under pine and bi rch, but rates of net N mineralization, nitrification and denitrification d id not differ under different seedlings. The stimulative effect of pine and especially birch on soil microbes was po ssibly due to them having more roots and releasing more root exudates to so il. There were, however, indications that not only the length/mass of roots determined the changes in microbial activities, but also differences in ro ot activities per unit of root or in the quality of root exudates.