Biomass allocation and nitrogenase activity in Alnus tenuifolia: Responsesto successional soil type and phosphorus availability

Low soil phosphorus availability and leachates from poplar (Populus balsami fera L.) have been hypothesized to limit growth and nitrogen fixation of th inleaf alder (Alnus tenuifolia Nutt.) in primary successional forests of th e Tanana River floodplain (interior Alaska). This greenhouse study isolates the effects of P availability and soil type (successional stage) on alder seedling growth and N-2 fixation (acetylene reduction). We grew seedlings i n soil cores collected from early-successional alder stands and mid-success ional poplar stands (poplar overstories with alder understories) which had been untreated or P fertilized in a field experiment. Total nodule biomass and nitrogenase activity per plans (measured at harvest) were similar among alders in both the alder and poplar soil. Alders grew larger in poplar soi l,but nitrogenase activity per gram of nodule was higher among alders grown in alder soil. Alders in fertilized soils grew larger than controls and in creased biomass allocation to nodules, resulting in much higher nitrogenase activity per plant, but nitrogenase activity per gram of nodule did not di ffer. Fertilization had a smaller effect on total plant biomass and total n odule biomass in alders grown in poplar soil than in alder soil. Results su ggest that low soil P could limit alder growth and N-2 fixation in these fl oodplain forests, but poplar soil is unlikely to limit growth and N-2 fixat ion relative to alder soil at natural levels of P.