Ammonium uptake by field-grown Eriophorum vaginatum roots under laboratory and simulated field conditions

Nitrogen (N) deficiencies in tundra ecosystems could be caused, in part, by the kinetics of root N uptake. The objectives of this study were to quantify NH₄ uptake by field-grown excised roots of Eriophorum vaginatum I. under controlled NH₄ concentrations (0-250 μmol I^-1) and temperatures (5-20°C) and to evaluate this laboratory derived model as a means of estimating field NH₄ uptake. There was no consistent temperature effect on root NH₄ uptake which suggests a relative in-sensitivity of E. vaginatum roots to short-term temperature fluctuations. The Michaelis-Menten equation parameters for NH₄ uptake were V_max= 22.1 μmol h^-1 g^-1 and K_m= 191 μmol I^-1. Using field NH₄ concentrations, field E. vaginatum root biomass data, and the Michaelis-Menten equation, an estimate was made of NH₄ uptake over a 42 day period; this estimate of NH₄ uptake accounted for 28% of the net incorporation of N into leaves and roots which is a reasonable estimate for E. vaginatum which relies primarily on N retranslocation for supplying new leaves and roots. Major uncertainties in field N uptake rates, model parameterization, and site characterization preclude an accurate model validation and indicate research areas most in need of future study.