Molybdate-reactive phosphorus losses in surface and drainage waters from permanent grassland

Although the loss of P in agricultural runoff can provide freshwater eutrophication, little information is available on losses from permanent grasslands. Thus, 14 lysimeter plots (each of 1 ha), established in 1982 on a clay loam soil under grazed grassland management, were monitored between 1989 and 1991 for release of molybdate-reactive phosphorus (MRP). The plots enabled the daily measurement of discharge through field drains (to 85 cm) or by surface lateral runoff (to 30 cm). Sward type was either perennial ryegrass (Lolium perenne L.) or a perennial ryegrass-clover (Trifolium sp.) mixture. The plots were grazed continuously by beef cattle and received either 200 or 400 kg N ha−1 yr−1 in various combinations. Having collected >7500 data points during the 3 yr, it was found that there was no significant difference between the effects of either agronomic or drainage variables (P > 0.05) within years. However, there was a significant difference (P < 0.05) between drainage variables when the data from the 3 yr were combined, as there were larger MRP losses from undrained treatments. There was no relationship between concentration of MRP and drainage volume. The MRP concentrations ranged from 0.01 to 0.7 mg L−1 with an average concentration of 0.02 mg L−1. Losses of MRP as great as 0.4 kg ha−1 yr−1 were measured in surface lateral runoff from undrained plots and 0.2 kg ha−1 yr−1 in water percolating through field drains. The data show this type of grassland system may have the potential to be an important diffuse source of P to fresh waters.