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Research Articles: "Doc2b is a high-affi nity Ca 2+ sensor for spontaneous neurotransmitter release" by A. J. Groffen et al. (26 March, p. 1614). Several author affi liations were not footnoted properly; three corrected affi liations follow.

A model of nutrient uptake was used to determine the factors that control the uptake of ammonium, glycine, and nitrate for an important arctic sedge, Eriophorum vaginatum, because the factors that regulate the nitrogen (N) uptake of plants in natural ecosystems are not well understood and a growing number of studies suggest that organic forms of N, including glycine, are an important source of N for E. vaginatum and other plants. E. vaginatum was selected as an exemplary system to explore nitrogen (N) uptake of a native species in situ, because it appears to be strongly N limited in the field, its N nutrition has been extensively studied, and its root growth habits make it well suited to modeling nutrient uptake. The model accounts for N supply from microbial mineralization and other sources, flux of N through the soil to the root surface, and uptake by the plant root. We included glycine in our simulations of N uptake because laboratory experiments have shown that E. vaginatum can use glycine and other low-molecular-mass amino acids as a source of N. However, the extent to which glycine contributes to the N nutrition of E. vaginatum in the field is unknown.Based on these simulations we find that: (1) Ammonium, glycine, and nitrate could all potentially make significant contributions to the N nutrition of E. vaginatum. The relative contribution of glycine is difficult to assess, because its behavior in the soil has not been characterized. However, glycine and ammonium contributed in roughly equal proportions for most model parameterizations.(2) The importance of factors that regulated modeled ammonium and glycine uptake can be ranked as follows: supply rate Ͼ [soil factors (buffer capacity and diffusion coefficient) ϭ root density] Ͼ root uptake kinetics. Supply rate was the only factor that regulated nitrate uptake. These simulation results and other considerations suggest that N uptake kinetics of roots and soil-solution concentrations may not be reliable predictors of whole-plant N uptake in tundra or other natural ecosystems.

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