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

The application of fertilizers and manure in excess of plant requirements h as resulted in an accumulation in soil phosphorus (P), and increased potent ial for P loss, To develop P-based catchment management plans, we need to b e 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 smal l catchment of mixed landuse (cereal crops (Triticum aestivum and Hordeum s ativum), root crops (Solanum tuberosum and Brassica sp.), grassland (Phleum pratense) and woodland (largely Castanea sativa)), in south Devon, UK. Thi s 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 discharg e 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 se asonal variation, with a late summer maximum and late winter minimum. Olsen P and CaCl2-P were related by a quantity-intensity relationship. Above a c ertain value of Olsen P, termed the change point, CaCl2-P increased more pe r unit Olsen P than below this point. The change point remained virtually c onstant 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 fo r 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 S RP 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 applicat ions 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 basi s of simple environmental management planning at the catchment scale. (C) 2 000 Elsevier Science B.V. All rights reserved.