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

¹³NO₃^– and ¹³NH₄⁺ 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.] Franco) at 0·1 and 1·5 mol m^–3 external NO₃^– or NH₄⁺ concentrations ([NO₃^–]_o or [NH₄⁺]_o, 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 ¹³N absorbed was translocated to the shoot during 57 min of ¹³N loading and elution, compared with less than 10% in the conifers. At 0·1 mol m^–3[NH₄⁺]_o, influx and net flux 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 ¹³N 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.