SEASONAL UPTAKE AND ALLOCATION OF PHOSPHORUS IN ERIOPHORUM-VAGINATUM L MEASURED BY LABELING WITH P-32

Seasonal measurements of phosphorus (P) pools in the tussock-forming sedge Eriophorum vaginatum L. indicated low annual P uptake, but pronounced fluxes of P to growing biomass from below-ground stores in spring, and a return in autumn. Additions of P-32 to tussocks in early spring and mid summer confirmed this deduction. Addition of P-32 caused a rapid incorporation of about 10 % of the labelled P in the tillers within a few days. Subsequent uptake was negligible, presumably because the P-32 not immediately absorbed was fixed in the soil and unavailable to plants. This demonstrates that nutrient pulses were more important than steady-state mineralization in supporting P uptake by Eriophorum vaginatum. After the P-32 additions, roots initially retained 60 % of the total P-32 taken up. During the following weeks, part of the P-32 was transported slowly to stem bases and leaf sheaths, the principal storage organs. A lower proportion was transported to the leaf blades. We suggest that storage and recirculation of nutrients are the main sources of the annual nutrient supply to growth of E. vaginatum under normal field conditions, where P fixation in the soil and low rate of P diffusion towards the roots limit P uptake from soil except during periods of pulsed release.