Phosphorus forms and concentrations in leachate under four grassland soil types

The transfer of P in water draining from agricultural land can contribute t o eutrophication and the growth of toxic algae. Traditionally, research has focused on particulate P transfer in surface pathways, with transfer by su bsurface pathways perceived as negligible, We investigated this by monitori ng P in leachate draining through large-scale monolith lysimeters (135 cm d eep, 80 cm diam.) installed in a field site in southwest England. The lysim eters were taken from four grassland soil types with a range of textures (s ilty clay-sand) and extractable-P contents (15-75 mg kg(-1) NaHCO3 extracta ble 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 i n the leachate routinely exceeded 100 mu g L-1 and remained relatively stab le throughout the drainage season, except during the late spring period whe n maximum concentrations >200 mu g L-1 were detected from all soil types. P hysically, most of the leachate P was dissolved (<0.45 mu m), although 21 t o 46% occurred in the particulate (>0.45 pm) size fraction, most notably fr om the sandy-textured soils. Chemically, the leachate was dominated by reac tive (inorganic) P from all soil types (62-71%), although a large proportio n was in unreactive (organic)P forms (29-28%). Reactive P occurred mainly i n the <0.45 pm fraction, while unreactive P was predominantly in the >0.45 fraction. Unreactive P in the <0.45 pm fraction was greatest during the spr ingtime (April-May), probably reflecting microbiological turnover and relea se of P in the soil. Our results indicate that (i) subsurface P transfer fr om soil to surface water can occur at concentrations that could cause eutro phication and (ii) unreactive and >0.45 pm P forms are important in subsurf ace P transfer.