SOIL-PHOSPHORUS EXTRACTABILITY AND UPTAKE IN A CIRSIO-MOLINIETUM FEN-MEADOW AND AN ADJACENT HOLCUS-LANATUS PASTURE ON THE CULM MEASURES, NORTH DEVON, UK

The seasonal pattern of extractable soil P and soil solution P was det ermined for topsoil samples taken from a Cirsio-Molinietum fen-meadow community, and also for an adjacent agriculturally improved pasture in Devon, UK. Phosphorus concentration was investigated using three meth ods during 1993; Olsen's reagent (sodium bicarbonate) for soil P, cent rifuging and polymeric suction cups for soil solution P. Solutions wer e extracted at two-weekly intervals during the spring and at monthly i ntervals during the summer. Phosphorus removed from both sites by cent rifuging was in the range 5-30 mu g dm(-3) soil, in comparison with 1- 6 mg kg(-1) for Olsen P. Olsen P on the Cirsio-Molinietum was signific antly lower than on the improved pasture in mid-March and mid-June. Th e concentration of P in soil solution removed by suction cups was belo w the level of detection (<2 mu g dm(-3)) on the Cirsio-Molinietum. Su ction cup P on the improved pasture peaked in April and May at 16-19 m u g dm(-3) soil. Herbage yield and P concentration were measured throu ghout the growing season. Above-ground standing crop was greater on th e Cirsio-Molinietum than the improved pasture at the beginning of the grazing season, because of the large litter component on the former. T otal P content of this material was only 1.81 kg P ha(-1) on the Cirsi o-Molinietum, in comparison with 5.58 kg P ha(-1) on the improved past ure. Phosphorus concentration of plant material obtained by repeated d efoliation was approximately 1.0 mg P g(-1) DM on the Cirsio-Molinietu m, whereas a clear seasonal trend, peaking at 3.5 mg P g(-1) DM, was o bserved on the improved pasture. phosphorus concentration of litter, g rass and sedge from ungrazed plots on the Cirsio-Molinietum indicated distinct seasonal variations for the grass, with no seasonal pattern f or the sedge component. (C) 1998 Elsevier Science B.V. All rights rese rved.