A MODEL OF NITROGEN UPTAKE BY ERIOPHORUM-VAGINATUM ROOTS IN THE FIELD- ECOLOGICAL IMPLICATIONS

A model of nutrient uptake was used to determine the factors that cont rol the uptake of ammonium, glycine, and nitrate for an important arct ic sedge, Eriophorum vaginatum, because the factors that regulate the nitrogen (N) uptake of plants in natural ecosystems are not well under stood 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 expl ore nitrogen (N) uptake of a native species in situ, because it appear s to be strongly N limited in the held, its N nutrition has been exten sively studied, and its root growth habits make it well suited to mode ling nutrient uptake. The model accounts for N supply from microbial m ineralization and other sources, flux of N through the soil to the roo t surface, and uptake by the plant root. We included glycine in our si mulations of N uptake because laboratory experiments have shown that E . vaginatum can use glycine and other low-molecular-mass amino acids a s a source of N. However, the extent to which glycine contributes to t he N nutrition of E. vaginatum in the field is unknown. Based on these simulations we find that: (1) Ammonium, glycine, and nitrate could al l 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 mo st model parameterizations. (2) The importance of factors that regulat ed modeled ammonium and glycine uptake can be ranked as follows: suppl y rate > [soil factors (buffer capacity and diffusion coefficient) = r oot density] > root uptake kinetics. Supply rate was the only factor t hat regulated nitrate uptake. These simulation results and other consi derations suggest that N uptake kinetics of roots and soil-solution co ncentrations may not be reliable predictors of whole-plant N uptake in tundra or other natural ecosystems.