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

Abstract

ABSTRACTRoot NO3− and NH4+ 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. NO3− and NH4+ 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. NO3− influx via HATS was inducible (IHATS); nitrate pre‐treatment resulted in 8–10‐fold increases in the Vmax for influx in both species. By contrast, HATS for NH4+ were entirely constitutive. In both species, Vmax values for NH4+ influx were higher than those for NO3− uptake; the differences were larger in pine (6‐fold) than aspen (1·8‐fold). In aspen, the Km for NH4+ influx by HATS was approximately 3‐fold higher than for IHATS NO3− influx, while in pine the Km for IHATS NO3− influx was approximately 3‐fold higher than for NH4+ influx. The aspen IHATS for NO3− influx appeared to be more efficient than that of pine (Vmax values for aspen being approximately 10‐fold higher and Km values being approximately 13‐fold lower than for pine). By contrast, only small differences in values for the NH4+ 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.