Phosphorus Forms and Concentrations in Leachate under Four Grassland Soil Types

Abstract

The transfer of P in water draining from agricultural land can contribute to eutrophication and the growth of toxic algae. Traditionally, research has focused on particulate P transfer in surface pathways, with transfer by subsurface pathways perceived as negligible. We investigated this by monitoring P in leachate draining through large‐scale monolith lysimeters (135 cm deep, 80 cm diam.) installed in a field site in southwest England. The lysimeters were taken from four grassland soil types with a range of textures (silty clay–sand) and extractable‐P contents (15–75 mg kg −1 NaHCO 3 extractable P) and leachate was sampled over two drainage seasons. Export of total P was <0.5 kg ha −1 yr −1 for all soil types. Concentrations of total P in the leachate routinely exceeded 100 μg L −1 and remained relatively stable throughout the drainage season, except during the late spring period when maximum concentrations >200 μg L −1 were detected from all soil types. Physically, most of the leachate P was dissolved (<0.45 μm), although 21 to 46% occurred in the particulate (>0.45 μm) size fraction, most notably from the sandy‐textured soils. Chemically, the leachate was dominated by reactive (inorganic) P from all soil types (62–71%), although a large proportion was in unreactive (organic) P forms (29–38%). Reactive P occurred mainly in the <0.45 μm fraction, while unreactive P was predominantly in the >0.45 fraction. Unreactive P in the <0.45 μm fraction was greatest during the springtime (April–May), probably reflecting microbiological turnover and release of P in the soil. Our results indicate that (i) subsurface P transfer from soil to surface water can occur at concentrations that could cause eutrophication and (ii) unreactive and >0.45 μm P forms are important in subsurface P transfer.