Factors affecting uranium and thorium fractionation and profile distribution in contrasting arable and woodland soils

This work investigated the vertical distribution and fractionation of uranium and thorium in soils under contrasting arable and woodland sites based on the same parent material. The effect of long-term application of phosphate fertilizer on U and Th concentrations in the arable soil was also assessed. The arable surface soils contained higher amounts of U compared with underlying layers and the woodland soil profile; by contrast, the Th distributions were very similar in both arable and woodland soils. The U and phosphate concentration profiles within the arable soil were broadly similar; U was strongly associated with Ca (r = 0.94) and soil P content (r = 0.86) whereas the U concentration profile in the woodland soil was virtually uniform with depth. The 'excess' U in arable soil, associated with long term P fertilizer application, was approximately 2.5 kg ha− 1 with 80% in the top 30 cm of the soil profile. A sequential extraction technique was used to fractionate U and discriminate between 'reactive' (non-residual) and 'residual' forms of U. The reactive U was adsorbed, mainly by organic matter and Fe-Mn oxides in the arable topsoil and there was a very strong relationship between reactive U and soil phosphate content in the arable soil (r = 0.99, P < 0.001). The 'excess' U accumulated in the arable topsoil was also considerably more reactive than the co-existing native U. Thus the reactive fraction of the excess U ranged from 29 to 42% compared with 14 to 15% reactivity of the native U. Thorium in both soils showed a very consistent, and low, reactivity down the soil profile at about 4% of total soil Th content (96% residual), bound almost completely to humus.