Variation of phosphorus loss from a small Catchment in south Devon, UK

The application of fertilizers and manure in excess of plant requirements has resulted in an accumulation in soil phosphorus (P), and increased potential for P loss, To develop P-based catchment management plans, we need to be able to estimate the impact of soil P on water draining a catchment. A 12-month investigation (August 1997-July 1998) determined the temporal change of soil P forms and soluble reactive P (SRP) in stream discharge in a small catchment of mixed landuse (cereal crops (Triticum aestivum and Hordeum sativum), root crops (Solanum tuberosum and Brassica sp.), grassland (Phleum pratense) and woodland (largely Castanea sativa)), in south Devon, UK. This included monthly sampling of soils for sodium bicarbonate extractable P (Olsen P), calcium chloride extractable P (CaCl2-P) on wet and air-dry soil, organic carbon and pH. Also available were weekly data for stream discharge and SRP concentration during 1987-1989 and 1994-1998, which enabled an 8-year mean to be calculated fur each month. All forms of soil P exhibited seasonal variation, with a late summer maximum and late winter minimum. Olsen P and CaCl2-P were related by a quantity-intensity relationship. Above a certain value of Olsen P, termed the change point, CaCl2-P increased more per unit Olsen P than below this point. The change point remained virtually constant throughout the year, nr:ver deviating more than 5 mg kg(-1) from a mean value of 31 mg kg(-1) Olsen P, Changes in stream SRP concentrations for the monthly means for 8 years data were correlated only with CaCl2-P from dry soil. Plots of cumulative SRP export against cumulative discharge over the 8-year data set suggested that SRP loss was limited by the supply of SRP from the soil matrix. Olsen P for root and cropping soils was twice that needed for maximum yields. Thus, to reduce SRP loss, P fertilizer applications should be stopped to allow Olsen P to decrease below the change point. The use of CaCl2-P and the change point has the potential to form the basis of simple environmental management planning at the catchment scale. (C) 2000 Elsevier Science B.V. All rights reserved.