EFFECTS OF BALSAM POPLAR (POPULUS-BALSAMIFERA) TANNINS AND LOW-MOLECULAR-WEIGHT PHENOLICS ON MICROBIAL ACTIVITY IN TAIGA FLOODPLAIN SOIL - IMPLICATIONS FOR CHANGES IN N-CYCLING DURING SUCCESSION

The transition from alder (Alnus tenuifolia) to balsam poplar (Populus balsamifera) is a critical turning point in primary succession on riv er floodplains in interior Alaska. Associated with the change in plant species are large changes in N cycling. N-fixation and nitrification decrease and the system becomes N-limited, with NH4+ dominating the in organic N pool. Balsam poplar leaves contain large quantities of tanni ns and low molecular weight phenolic compounds. We evaluated the effec t of these compounds on microbial respiration and N cycling in laborat ory assays on soils from an alder-dominated site. Plant compounds were purified and applied to silica gel as an inert carrier. Both tannins and phenolics caused net N-immobilization over a 30-day assay. However , tannins inhibited respiration while phenolics stimulated it. There w ere no specific effects on nitrification. Thus, tannins acted as a gen eral microbial inhibitor, while phenolics acted as a growth substrate. By inhibiting mineralization while stimulating immobilization, poplar secondary compounds may reduce soil N-availability during the transit ion between alder and poplar stages in succession.