GROWTH OF PAPER BIRCH (BETULA-PAPYRIFERA) SEEDLINGS INCREASES SOIL AVAILABLE C AND MICROBIAL ACQUISITION OF SOIL-NUTRIENTS

We compared the flux of energy and nutrients in a mineral forest soil in which paper birch (Betula papyrifera) seedlings had grown with soil s that had been exposed to one of five other tree species over a 22 we ek growing period. Soil basal respiration rate, metabolic quotient, so il available C (AC), and the affinity of soil microorganisms for subst rate-C left in the soil after harvest all increased significantly, in soils treated with birch root systems. Concentrations of AC in birch-t reated soils related to the energy-only limited microbial biomass (MBE ), but not the nutritionally limited microbial biomass (MEN). Amounts of rhizosphere activity, described as root-supported MBE per unit root and per unit fine root, were one order of magnitude higher in the bir ch rhizosphere. Plant uptake of soil-N during the growing period was h igh while the soil mineral-N pool was low in birch experimental units relative to those of other species, suggesting that birch competed wel l against soil microorganisms for available mineral-N. Anaerobic N min eralization rates were significantly higher while the MB(N)-to-MB(E) r atio, which describes the degree of nutritional limitation of the micr obial biomass, was significantly lower in birch-treated soils. Signifi cant negative correlations were found between the MB(N)-to-MB(E) ratio and both AC and MB(E). These results suggest that high amounts of roo t labile C compounds in conjunction with rapid mineral-N uptake by bir ch roots can stimulate microbial communities to acquire nutrients from the native soil.