NITROGEN TURNOVER AND AVAILABILITY DURING SUCCESSION FROM ALDER TO POPLAR IN ALASKAN TAIGA FORESTS

Primary succession on the Tanana river floodplain progresses from alder, with C-limited microbes and rapid nitrification, to poplar, with N-limited microbes and little nitrification. To determine the mechanisms controlling this shift, a reciprocal soil transplant experiment was conducted between alder and poplar sites. Mineralization rates (gross and net) and nitrification potentials were measured periodically over 1 year. Nitrification potentials of both soils were higher in the alder site, but transplanted soils showed higher gross rates of N turnover than soils in their native site. Thus, C additions from poplar alleviated C limitation in alder soils, accelerating N turnover, but with a net reduction in N availability to nitrifiers. Transplanting poplar soil to the alder site alleviated N limitation, accelerating N turnover while increasing net N availability. The changes in N cycling between alder and poplar were therefore controlled by changes in N turnover and availability.