The long-term accumulation of bioavailable uranium in soil due to the use of mineral phosphate fertilizers

Cadmium (Cd) and uranium (U) are toxic metals that are present at relatively high concentrations in mineral phosphate (P) fertilisers. Long-term use of P fertilisers may enrich soils with both metals, potentially enriching food crops. In 2019, European limits for Cd in fertilisers have been adopted to avoid such enrichments. The limits were derived based on Cd mass balances in soil. No such limits have yet been defined for uranium. The derivation of the Cd limits in fertilisers has been criticised because they might have overestimated Cd losses from soil by leaching. If Cd leaching is indeed lower than predicted, Cd limits in fertilisers should be revised and should become even more strict. This work was undertaken to make more accurate estimates of leaching losses of Cd and U in agricultural soils and to use that information to revise risk assessments of these metals in P fertilisers. By examining long-term P application trials, it was found that Cd accumulation in soils due to P fertiliser application is highly variable and relatively rare compared to U accumulation. Column experiments showed that colloidal transport enhances Cd and, mainly, U leaching. Most routinely used soil solution extraction methods partly exclude these colloids and, hence, may underestimate leaching. The models used for 2019 Cd limits were based on such routine methods but mainly used soils that had been air dried, rewetted and incubated. Remarkably, the soil drying and rewetting was here shown to stimulate soil nitrification that mobilises Cd considerably compared to pore water Cd in field fresh soils. Taken together, there is sufficient evidence to confirm the criticism that the traditional models for predicting Cd in solution might overestimate the long-term average Cd leaching in the field. The stand-still principle was finally revisited with the new information, thereby predicting lower Cd leaching but still predicting a small net Cd loss at EU scale on the long-term, confirming the new empirical evidence derived from the long-term P application trials. A more stringent EU-wide limit for Cd in fertilisers may not be the most effective approach. Instead, localised strategies, considering factors such as P-application rates and soil characteristics, might be more practical for mitigating Cd accumulation risks. Overall, this research highlights the high variability in fertiliser derived Cd accumulation and the complex dynamics of metal leaching in soils.