A comparative kinetic analysis of nitrate and ammonium influx in two early‐successional tree species of temperate and boreal forest ecosystems

Root NO 3 − and NH 4 + influx systems of two early‐successional species of temperate (trembling aspen: Populus tremuloides Michx.) and boreal (lodgepole pine: Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) forest ecosystems were characterized. NO 3 − and NH 4 + influxes were biphasic, consisting of saturable high‐affinity (HATS) and constitutive non‐saturable low‐affinity transport systems (LATS) that were evident at low and relatively high N concentrations, respectively. NO 3 − influx via HATS was inducible (IHATS); nitrate pre‐treatment resulted in 8–10‐fold increases in the V max for influx in both species. By contrast, HATS for NH 4 + were entirely constitutive. In both species, V max values for NH 4 + influx were higher than those for NO 3 − uptake; the differences were larger in pine (6‐fold) than aspen (1·8‐fold). In aspen, the K m for NH 4 + influx by HATS was approximately 3‐fold higher than for IHATS NO 3 − influx, while in pine the K m for IHATS NO 3 − influx was approximately 3‐fold higher than for NH 4 + influx. The aspen IHATS for NO 3 − influx appeared to be more efficient than that of pine ( V max values for aspen being approximately 10‐fold higher and K m values being approximately 13‐fold lower than for pine). By contrast, only small differences in values for the NH 4 + HATS were evident between the two species. The kinetic parameters observed here probably result from adaptations to the N availabilities in their respective natural habitats; these may contribute to the distribution and niche separation of these species.