Extracellular Acid Phosphatase Activities in Eriophorum vaginatum Tussocks: A Modeling Synthesis

Abstract

Analyses of Eriophorum vaginatum tussocks provided mass and kinetic parameters for a Michaelis-Menten model of phosphatase activities in Alaskan tussock tundra. This model was used to simulate the temporal patterns of phosphatase activities, given a 90-d thawing season and organic phosphorus concentrations of 30 ,LM in the first and last 10-d intervals; 15 uM at other times. Results indicated that about 28% of the total annual tussock activity (155 mg P released) occurred during the brief period of high substrate availability in autumn; little occurred in spring because most of the tussock was frozen and live root mass was low. Phosphatases associated with living roots of E. vaginatum were responsible for about 4% of the total activity in tussocks (ca. 6 mg P), which is almost twice the annual plant demand (ca. 3.5 mg). These results suggest that (1) E. vaginatum may obtain much of its phosphorus requirement from the activities of root surface phosphatases, and (2) the timing of maximum plant phosphorus uptake (late in year) and growth (early in year) are asynchronous, i.e., E. vaginatum integrates nutrient availabilities across years. and is regrown each summer. Therefore, living root mass increases throughout the growing season. The total depth of soil thaw also increases during this period, expanding the potential rooting volume and stimulating enzyme activities. In addition, soil solution organic phosphorus concentrations change with time. For these reasons, any assessment of tussock phosphatase activities must account for these dynamically changing factors. The goals of this study were to evaluate the potential contributions of E. vaginatum root surface phosphatases to (1) total rhizosphere phosphatase activity and (2) satisfying plant phosphorus requirement. A combination of experimental and modeling approaches was used to address these objectives. The various components of E. vaginatum tussocks (e.g., live roots, dead roots, soil, etc.) were separated, quantified, and their associated phosphatase activities assayed. This information was then used to develop a model of rhizosphere enzyme activity, including the effects of soil thawing, root growth, and fluctuating organic phosphorus concentrations. Simulations were performed to elucidate the dynamics of tussock phosphatase activities throughout the growing season.