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 soils that had been exposed to one of five other tree species over a 22 week growing period. Soil basal respiration rate, metabolic quotient, soil available C (AC), and the affinity of soil microoganisms for substrate-C left in the soil after harvest all increased significantly, in soils treated with birch root systems. Concentrations of AC in birch-treated soils related to the energy-only limited microbial biomass (MBE), but not the nutritionally limited microbial biomass (MBN). 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 birch rhizosphere. Plant uptake of soil-N during the growing period was high while the soil mineral-N pool was low in birch experimental units relative to those of other species, suggesting that birch competed well against soil microorganisms for available mineral-N. Anaerobic N mineralization rates were significantly higher while the MBN-to-MBE ratio, which describes the degree of nutritional limitation of the microbial biomass, was significantly lower in birch-treated soils. Significant negative correlations were found between the MBN-to-MBE ratio and both AC and MBE. These results suggest that high amounts of root labile C compounds in conjunction with rapid mineral-N uptake by birch roots can stimulate microbial communities to acquire nutrients from the native soil.