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TECHNICAL REPORTS

Landscape and Watershed Processes

Spatial Variability of Soil Phosphorus in Relation to the Topographic Index and Critical Source Areas

Sampling for Assessing Risk to Water Quality

^a Environmental Science Department, University of Lancaster, Lancaster, LA1 4YQ, UK
^b Soil Science and Environmental Quality Team, Institute of Grassland and Environmental Research, North Wyke Research Station, Okehampton, Devon, EX20 2SB, UK
^c National Soil Resources Institute, Cranfield University, North Wyke Research Station, Okehampton, Devon, EX20 2SB, UK
^d National Soil Resources Institute, Cranfield University, Silsoe, Bedfordshire, MK45 4DT, UK

^* Corresponding author (phil.haygarth{at}bbsrc.ac.uk)

Received for publication October 27, 2004. A measure of soil P status in agricultural soils is generally required for assisting with prediction of potential P loss from agricultural catchments and assessing risk for water quality. The objectives of this paper are twofold: (i) investigating the soil P status, distribution, and variability, both spatially and with soil depth, of two different first-order catchments; and (ii) determining variation in soil P concentration in relation to catchment topography (quantified as the "topographic index") and critical source areas (CSAs). The soil P measurements showed large spatial variability, not only between fields and land uses, but also within individual fields and in part was thought to be strongly influenced by areas where cattle tended to congregate and areas where manure was most commonly spread. Topographic index alone was not related to the distribution of soil P, and does not seem to provide an adequate indicator for CSAs in the study catchments. However, CSAs may be used in conjunction with soil P data for help in determining a more "effective" catchment soil P status. The difficulties in defining CSAs a priori, particularly for modeling and prediction purposes, however, suggest that other more "integrated" measures of catchment soil P status, such as baseflow P concentrations or streambed sediment P concentrations, might be more useful. Since observed soil P distribution is variable and is also difficult to relate to nationally available soil P data, any assessment of soil P status for determining risk of P loss is uncertain and problematic, given other catchment physicochemical characteristics and the sampling strategy employed.

Abbreviations: CSA, critical source area • NSI, National Soil Inventory • WSRP, water-soluble reactive phosphorus • WSTP, water-soluble total phosphorus

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