MOLYBDATE-REACTIVE PHOSPHORUS LOSSES IN SURFACE AND DRAINAGE WATERS FROM PERMANENT GRASSLAND

Although the loss of P in agricultural runoff can provide freshwater e utrophication, little information is available on losses from permanen t grasslands. Thus, 14 lysimeter plots (each of 1 ha), established in 1982 on a clay loam soil under grazed grassland management, were monit ored between 1989 and 1991 for release of molybdate-reactive phosphoru s (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 contin uously by beef cattle and received either 200 or 400 kg N ha(-1) yr(-1 ) in various combinations. Having collected >7500 data points during t he 3 yr, it was found that there was no significant difference between the effects of either agronomic or drainage variables (P >0.05) withi n years. However, there was a significant difference (P <0.05) between drainage variables when the data from the 3 yr were combined, as ther e were larger MRP losses from undrained treatments. There was no relat ionship between concentration of MRP and drainage volume. The MRP conc entrations ranged from 0.01 to 0.7 mg L(-1) with an average concentrat ion of 0.02 mg L(-1). Losses of MRP as great as 0.4 kg ha(-1) yr(-1) w ere measured in surface lateral runoff from undrained plots and 0.2 kg ha(-1) yr(-1) in water percolating through field drains. The data sho w this type of grassland system may have the potential to be an import ant diffuse source of P to fresh waters.