Controls on the phosphorus content of fine stream bed sediments in agricultural headwater catchments at the landscape-scale

There have been no landscape-scale assessments which quantify the relative importance of the organic and mineral properties of BS (bed sediment) and associated catchment characteristics (geology, land cover and mean topsoil phosphorus (P) content) for BSP concentration. Mid infra red diffuse reflectance spectrometry was applied to estimate the quantities of organic matter, dithionite extractable aluminium- (Al_d) and iron (Fe_d), kaolinite, dioctahedral clay and mica (D&M) minerals in 1052 snapshot samples of fine (<150 μm) BS in small to medium-sized (5-55 km²) agricultural headwater catchments across a large area (15 400 km²) of central England. Analyses included estimates of BS specific surface area, cerium (Ce) concentrations (enriched in P-bearing apatite and P-fertilsers), and catchment average topsoil P content.Simple linear regression demonstrated that the proportion of variance in BSP explained by specific components of BS across all catchments declined in the following order: Al_d > Fe_d > topsoil P = kaolinite = residual iron> organic matter = Ce> D&M > mineral SSA. No single component accounted for more than 36% of the variance in BSP. Multiple regression - including a classification of bedrock lithology and proportions of arable and grassland by area - accounted for 61.9% of the variance in BSP. The proportion of arable and grassland by area in each catchment was also a statistically significant predictor of BSP. Across this large region - with widely differing geology and soils - Fe_d in BS is more strongly associated with kaolinite than D&M minerals because iron-oxyhydroxides and kaolinite form contemporaneously during pedogenesis. The SSA of fine bed sediments is largely determined by catchment area, fitted accurately using a power function.