Uranium accumulation in agricultural soils as derived from long-term phosphorus fertilizer applications

Abstract

It is well known that uranium (U) in mineral phosphorus (P) fertilizers may accumulate in agricultural soils; yet, this U accumulation occurs at different rates, likely depending on the type of fertilizer used. To substantiate this assumption, the aims of my thesis were: i) to quantify the accumulation rates of fertilizer-derived U in different long-term agricultural field experiments with P fertilized soils of central Europe, and ii) to contrast this with data from long-term experimental sites on volcanic soils that require higher amounts of P fertilizers for optimal crop production, and iii) finally to explain the variations of U accumulation rates by an assessment of the formation mechanisms and stocks of U in major phosphate rocks (PRs) deposits of the world. Soil samples were taken from the surface soils and selected depth profiles of seven long-term experiment sites, i.e. at the grassland fertilization trials in Rengen (Germany), Park Grass (Rothamsted, UK), as well as in Geitasandur and Sámstaðir (Iceland), and the agricultural field experimental sites in Thyrow (Germany), Askov (Denmark), Broadbalk (Rothamsted, UK), Uranium concentrations were analyzed after microwave-assisted acid digestion by nitric acid or/and a complete digestion by lithium meta/tetraborate. In addition, I assessed U concentrations and the natural stable oxygen isotope compositions of phosphate (δ ¹⁸O_P) in PRs as potential indicators for the genesis of the U in PRs from different deposits all over the world. <br /> My results revealed a wide range of U accumulation rates in soils, ranging from 0-310 µg U kg ^-1 yr^-1 in the monitored fields. Uranium accumulation was small when the P fertilizers were derived from igneous PRs from Finland and Kola Peninsula, as used for sites in Askov (< 0.4 µg U kg^-1 yr^-1; <1.2 g ha^-1 yr^-1 (20 cm)) and Thyrow (0.6 µg U kg^-1 yr^-1; 2.3 g ha^-1 yr^-1 (24 cm)), respectively, or when basic slag was applied as used for the Rengen (1.2 µg U kg-1 yr-1; 1.3 g ha^-1 yr^-1 (10 cm)) site. Higher U accumulation rates (3.4, 7.8 µg U kg^-1 yr^-1; 11.7, 21.9 g ha^-1 yr^-1 (23 cm)) were found at Rothemsted experiment stations (Broadbalk; Parkgrass), where P fertilizers used had been predominantly produced from PRs from North Africa. The most serious case of fertilizer-derived U accumulation (up to 310 µg U kg^-1 yr^-1; 33.2 g ha^-1 yr^-1 (5 cm)) was found in Icelandic agricultural soil as a consequence of both high U concentrations in the applied P fertilizers (from an unknown PR source) and of large amounts of P fertilizer application. Overall, soil U concentrations will increase by 0.5 µg U kg^-1 (0-5.1 µg U kg^-1) soil for 1 kg P applied per hectare and there will be 2.7-11 g U ha^-1 yr^-1 input to the EU’s agricultural soil with 21.2 kg P (as P₂O₅) per hectare fertilization. To explain these variations of U concentrations in the world’s PRs, U concentrations in the PRs were discussed corresponding to their δ¹⁸O_P values. I found that there was a ‘co-evolutionary’ relationship between the U (U/P₂O₅ ratio) and the δ ¹⁸O_P value: the lower the δ ¹⁸O_P value of the PR was, the lower its U concentration was. In igneous PRs, low U concentration can be explained by the lack of secondary U enrichment processes after rock formation, whereas the low δ ¹⁸O_P values were resulted from limited isotope fractionation at high temperatures in the magma. In sedimentary PRs, on the other hand, the variations of U concentrations and δ ¹⁸O_P values were related to the geologic age at which PRs were formed. Generally, older sedimentary PRs (formed in Precambrian-Cambrian) exhibited lower U concentrations and lower δ ¹⁸O_P values than the younger ones (formed in Ordovician-Neogene), which were influenced by paleoclimate and paleographic features. <br /> In summary, the accumulation rates of fertilizer-derived U in agricultural soils were region-specific, depending on the source and the amount of P fertilizer applied. My data show that when applying P fertilizers with low U content, soil U concentration will remain at a non-critical level even at multi-centennial scale. However, fertilizer-derived U accumulation may pose an environmental issue at the places where large amounts of P fertilizers are needed for maximizing crop production (such as in Andosols). Selecting igneous PRs or ancient sedimentary PRs (formed in Paleozoic and Precambrian) as precursor materials for P fertilizer production is therefore crucial for minimizing potential U contamination risks and thus for sustainable agricultural management.