Influence of balsam poplar tannin fractions on carbon and nitrogen dynamics in Alaskan taiga floodplain soils

The feedbacks between plant and soil processes play an important role in dr iving forest succession. One poorly understood feedback mechanism is the in teraction between plant secondary chemicals and soil microbes. In the Alask an taiga, changes in nutrient cycling caused by balsam poplar (Populus bals amifera) secondary chemicals may affect the transition from alder (Alnus te nuifolia) to balsam poplar on river floodplains. We examined the effects of four poplar condensed tannin fractions on N cycling in alder and poplar so ils. Tannins were added to forest floor samples from both poplar and alder sites. Samples were incubated for 1 month in the laboratory with soil respi ration rates measured over the course of the incubation. At the end of the incubation we measured both act and gross nitrogen mineralization and nitri fication, microbial biomass C and N, and the activity of various exoenzymes . In all soils, tannin additions reduced N availability, however, the mecha nisms differed depending on the molecular weight of the tannin and the nati ve soil microbial community. Low molecular weight tannin fractions served a s a labile C source in poplar Oi, poplar Oc, and alder Oe horizons but were toxic to microbes in alder Oi. High molecular weight tannin fractions appe ared to act primarily by binding extracellular substrates and thus limiting C and N mineralization, with the strongest effects observed in the alder s oils. (C) 2001 Elsevier Science Ltd. All rights reserved.