NITROGEN ACQUISITION AND COMPETITIVE ABILITY OF KALMIA-ANGUSTIFOLIA L., PAPER BIRCH (BETULA-PAPYRIFERA MARSH.) AND BLACK SPRUCE (PICEA-MARIANA (MILL.) BSP) SEEDLINGS GROWN ON DIFFERENT HUMUS FORMS

Two species of boreal tree seedlings, paper birch (Betula papyrifera M arsh.) and black spruce (Picea mariana (Mill.) B.S.P.), and the ericac eous shrub Kalmia angustifolia L. were grown in pots with humus from a birch-dominated site and two spruce-Kalmia sites. Root systems intera cted with humus form in controlling soil-N cycling as well as energy a nd nutritional deficiencies of soil microorganisms. In general, Kalmia seedlings affected microbial dynamics and N cycling differently than birch and spruce seedlings did. Birch and spruce seedlings reduced gro ss N mineralization and immobilization rates, soil mineral-N pools and the amounts of NH4+-N accreted on buried cation exchange resins in al l three soils. Compared to birch and spruce seedlings, the growth of K almia resulted in significantly higher gross N mineralization rates, s oil mineral-N pools and resin-NH4+ accretion in soil from the fertile birch site. Gross N immobilization rates in all soils were generally h igher with Kalmia than with spruce or birch seedlings. All three speci es of seedlings acquired N from the birch site soil, whereas only Kalm ia seedlings acquired N from the two spruce-Kalmia site soils. Relativ e to control treatments, the amount of N mineralized anaerobically inc reased in the birch-site soil and decreased in the poor spruce-Kalmia site soil with all three species of seedlings. All seedlings increased the microbial biomass in the birch-site soil. Kalmia humus and Kalmia root systems increased microbial energy-deficiency and decreased micr obial nutritional deficiency compared to the other humus and seedlings used. Results are discussed in terms of each species' nutrient acquis ition mechanism and its competitive ability during secondary successio n.