Forms of phosphorus transfer in hydrological pathways from soil under grazed grassland

Phosphorus (P) from soil can impair the water quality of streams and lakes. We have studied the forms and pathways of its movement from soil to water using 1-ha plot lysimeters, managed as grazed grassland for 12 months in temperate South-west England. The water flow through three pathways, namely (i) surface plus interflow to 30 cm (on undrained soil), (ii) surface plus interflow to 30 cm (on a mole and tile drained soil), and (iii) mole and tile drains (to 85 cm), were gauged. Samples of water from each path were treated with various combinations of 0.45-μm filtration and sulphuric acid-persulphate digestion and molybdate reaction, to determine the different forms of P. The total P (TP) concentration was greatest in the surface plus interflow to 30 cm paths (means 232 and 152 μg l^–1), whereas the mean concentration in the drainage to 85 cm was 132 μg l^–1. This reflects the substantial enrichment of the Olsen-P extracts from the surface horizons, as extracts from the 0–2 cm layer were 10-fold more than below 45 cm. In all paths, the dissolved P comprised the greatest proportion of the P transferred, with dissolved reactive P being the dominant form. Draining land reduced the transfer of TP by about 30% (≈ 1 kg^–1 ha^–1 year^–1), because it can be sorbed as it flows through soil to drains. All these concentrations could cause eutrophication in surface waters.