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Phosphorus Loss in Drainflow from Intensively Managed Grassland Soils

The loss of P in subsurface drainage from lysimeters (0.5 ha), managed as either monoculture grass or grass-clover for the last 9 yr have been quantified. Both systems received two to three cattle slurry applications annually and were cut two to three times before being grazed by dairy cattle. Mineral fertilizer-P was applied only to the grass-clover (about 25 kg P ha -1 yr -1 ). After 9 yr, NaHCO 3 -soluble P in the topsoil (0-10 cm) averaged 38 and 47 mg P kg -1 for the grass and grass-clover respectively, giving an average increase of 1.0 mg Olsen-P kg -1 yr -1 in the grass-clover. Drainage-weighted molybdate-reactive phosphorus (MRP) and total phosphorus (TP) concentrations ranged from 0.16 to 0.38 mg P L -1 and 0.45 to 0.79 mg P L -1 , respectively during the 2-yr study period. The MRP and TP losses in subsurface-flow from the grass-clover (1.68-2.03 and 3.47-5.03 kg P ha -1 yr 1 , respectively) were significantly larger than those from the grass (1.27-1.34 and 2.97-3.58 kg P ha -1 yr -1 , respectively). Averaged across years and pastures, MRP accounted for 42% of the TP loss, while a non-MRP form accounted for 41% of the TP loss through field drains. Particulate-associated P represented about 17% of the TP loss. The P losses in subsurface runoff measured in the present study were much larger than previous estimates. The results also showed that, despite the subsoil having much larger P-sorption capacity than the topsoil, significant amounts of P losses could occur through preferential hydrological pathways.