The Role of Phosphorus in Vegetational Differentiation in a Small Valley Mire

Abstract

(1) Large variations in dissolved inorganic phosphate concentration (Psi) in water at Askham Bog, a small valley mire near York, are described. The site is domed, with a topographical range of about 40 cm and a soil pH range of about 3 units, corresponding with concentric zones of progressively more acidiphilous vegetation (poor-fen) towards the centre. (2) There was a close correlation between the contours for pH 4.5, and for 5 0,uM Psi and for the major boundary between basiphilous (rich-fen) and acidiphilous (poor-fen) vegetation, with low Psi associated with high pH. Analysis of phosphate and calcium potentials indicates that this is not due to co-precipitation. (3) Extremely high Psi concentrations (up to 60 ,uM) were found in two areas of low pH, associated with oakwood, the herb layer of which was dominated by Lonicera periclymenum and Rubus fruticosus, rather than the Betula-Molinia-Sphagnum association typical of the acid zone. (4) Concentrations of Psi and Ps. (soluble organically-based phosphorus) fluctuated during the summer, with lower concentrations when water-tables fell. (5) There were no differences in total peat-phosphorus concentrations between sites exhibiting a nearly twenty-fold difference in Psig but total P concentrations in peat were much greater above than below 35 cm depth, reflecting a change in peat type. (6) At a high-pH site, there were extremely high concentrations of extractable Ps0 in the peat above the water table despite very low concentrations of P (though very little Psi) in water-table water. At two acidic but Sphagnum-free sites, there was a small increase in Psi concentration above the water table, but low Pso concentrations and, at an acidic Sphagnum-dominated site, there was no change above the water-table. (7) A hypothesis is suggested to explain these differences, based on effects of waterlogging on mineralization and of pH on incorporation of mineralized phosphorus. It is possible that the course of succession depends upon this and that the invasion of Sphagnum introduces a radical shift in phosphorus cycling.