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

Low soil phosphorus availability and leachates from poplar (Populus balsamifera L.) have been hypothesized to limit growth and nitrogen fixation of thinleaf alder (Alnus tenuifolia Nutt.) in primary successional forests of the 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 in soil cores collected from early-successional alder stands and mid-successional 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 soil,but nitrogenase activity per gram of nodule was higher among alders grown in alder soil. Alders in fertilized soils grew larger than controls and increased biomass allocation to nodules, resulting in much higher nitrogenase activity per plant, but nitrogenase activity per gram of nodule did not differ. Fertilization had a smaller effect on total plant biomass and total nodule biomass in alders grown in poplar soil than in alder soil. Results suggest that low soil P could limit alder growth and N-2 fixation in these floodplain forests, but poplar soil is unlikely to limit growth and N-2 fixation relative to alder soil at natural levels of P.