Spatial variability of soil phosphorus in relation to the topographic index and critical source areas: sampling for assessing risk to water quality

• Published in: Journal of environmental quality 2005-11, Vol.34 (6), p.2263-2277
• Main Authors: Page, T, Haygarth, P.M, Beven, K.J, Joynes, A, Butler, T, Keeler, C, Freer, J, Owens, P.N, Wood, G.A
• Format: Article
• Language: English
• Subjects: Agricultural land; agricultural runoff; Agricultural watersheds; Agriculture; Agronomy. Soil science and plant productions; Applied sciences; Base flow; Biological and medical sciences; Catchments; Databases, Factual; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; Land use; Models, Theoretical; phosphorus; Phosphorus - analysis; Phosphorus - metabolism; Phosphorus content; Pollution; Pollution, environment geology; Risk assessment; Risk Factors; Soil; soil chemistry; Soil depth; Soil Pollutants - metabolism; soil water content; Soils; spatial distribution; spatial variation; Streambeds; topography; United Kingdom; Water Pollutants, Chemical - analysis; Water Pollutants, Chemical - metabolism; Water pollution; Water quality; Water quality measurements; Water Supply; watersheds
• ISSN: 0047-2425; 1537-2537
• DOI: 10.2134/jeq2004.0398

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.