A comparative study of fluxes and compartmentation of nitrate and ammonium in early-successional tree species

(NO3-)-N-13 and (NH4+)-N-13 compartmental analyses were carried out in seedling roots of trembling aspen (Populus tremuloides Michx,), lodgepole pine (Pinus contorta Dougl, ex Loud. var. latifolia Engelm,) and interior Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn,] France) at 0.1 and 1.5 mol m(-3) external NO3- or NH4+ concentrations ([NO3-](0) or [NH4+](0), respectively). At the lower [NO3-](o), the capacities and efficiencies of acquisition and accumulation of NO3-, based upon NO3- fluxes and cytoplasmic NO3- concentrations ([NO3-](c)), were in the order aspen >> Douglas-fir > pine. At 1.5 mol m(-3) [NO3-](o), the NO3- influx increased 18-fold in pine, four-fold in Douglas-fir and approximately 1.4-fold in aspen; in fact, at 1.5 mol m(-3) [NO3-](o), the NO3- influx in pine was higher than in aspen. However, at high [NO3-](o), efflux also increased in the two conifers to a much greater extent than in aspen. In aspen, at both [NO3-](o), approximately 30% of the N-13 absorbed was translocated to the shoot during 57 min of N-13 loading and elution, compared with less than 10% in the conifers, At 0.1 mol m(-3) [NH4+](o), influx and net nux were in the order: aspen > pine > Douglas-fir but the differences were much less than in NO3- fluxes. At 1.5 mol m(-3) [NH4+](o), NH4+ influx, efflux and [NH4+](c) greatly increased in aspen and Douglas-fir and, to a much lesser extent, in pine. In aspen, 29 and 12% of the N-13 absorbed was translocated to the shoot at 0.1 and 1.5 mol m(-3) [NH4+](o), respectively, compared with 5 to 7% in the conifers at either [NH4+](o). These patterns of nitrogen (N) uptake, particularly in the case of NO3-, and the observed concentration responses of NO3- uptake, reflect the availability of N in the ecological niches, to which these species are adapted.