The impact of pasture improvement on phosphorus concentrations in soils and streams in an upland catchment in Northern England

Upland pasture improvement historically undertaken to increase agricultural output may impair water quality due to increased phosphorus (P) transfer in land runoff. We monitored two adjacent headwater catchment areas of the River Rede in Northumberland with different proportions of previously improved grassland (7 versus 47% of total area) to assess potential P enrichment of their peaty topsoils and draining streams. Pastures had been improved during the 1970s and 1980s mostly by liming, fertilisation, pioneer cropping with stubble turnips for 2 years, reseeding with grass and clover and subsequently grazed by sheep. Fertiliser P inputs during and subsequent to improvement maintained available (Olsen) P concentrations in the soil (0–7.5 cm) at optimum levels for grass production (16–25 mg L−1), whilst unimproved areas contained only 4–6 mg L−1 of Olsen-P. Between 1994 and 1997, stream annual flow-weighted concentrations of dissolved reactive (inorganic) P (DRP), dissolved unreactive P (DUP) and particulate P (PP) were increased from 10, 29 and 39 μg L−1, respectively in the largely unimproved catchment to 21, 35 and 97 μg L−1, respectively in the catchment with 47% improved land. However, DRP remained a small proportion of TP in both catchments (ca. 12%) and only PP showed an increased proportional contribution (from 50% to 63%) due to an estimated four-fold increase in sediment P content. Our study suggests there is a direct link between pasture improvement, soil P accumulation and increased P fluxes in this upland stream. Despite annual total P loads approaching 1 kg ha−1, the small absolute increase in DRP concentrations suggested that a catchment with nearly 50% improved land could still meet current proposed standards for ‘good’ water quality in upland streams.