INCREASE IN SOLUBLE PHOSPHORUS TRANSPORTED IN DRAINFLOW FROM A GRASSLAND CATCHMENT IN RESPONSE TO SOIL-PHOSPHORUS ACCUMULATION

Because of the observed variability in soil available P (Olsen) conten ts, phosphorus budgets were used to predict changes in the soil P stat us of an intensively managed 6 ha grassland catchment in Northern Irel and. The P accumulation rate of approximately 24 kg/ha/y suggested an increase of soil available P (Olsen) of 1.0 mg P/kg/y. Soluble reactiv e phosphorus concentrations in drainflow measured on a daily basis for a two year period (January 1981 - December 1982) were compared with t he two year period January 1990 - December 1991. The median concentrat ion had increased by 10.0 mu g P/l in 1990/91 compared with 1981/82. T his difference was only apparent in mean concentrations for the two ti me periods, after data associated with high flow events, which were mo re frequent in 1981/82, were excluded from the comparison. This rate o f increase of 1.1 mu g P/l/y, which was interpreted as reflecting an i ncrease in soluble reactive phosphorus concentration in soil solution, is comparable to the increase in background soluble reactive phosphor us of 1.5+/-0.54 mu g P/l/y which was reported recently over a 17 year period from diffuse sources in the much larger (4400 km(2)) Northern Ireland catchment of Lough Neagh.