Fertilizer P-derived uranium continues to accumulate at Rothamsted long-term experiments

The application of phosphorus (P) fertilizers inevitably contributes to the accumulation of trace elements, such as ura-nium (U), in agricultural soils. The fertilizer-derived U accumulation was first reported in Rothamsted Research in 1979. In the present study, we expand this early key research by evaluating the fertilizer-derived U accumulation in topsoil (0-23 cm) from 1876 to the 2010s. We found that total U accumulation rates ranged from 2.8 to 6.1 mu g U kg(-1) yr(-1) at the Broadbalk and Park Grass, respectively, being similar to those observed 40 years ago. This highlights that U accumulation is still an ongoing process in Rothamsted. Fortunately, the proportion of fertilizer-derived U did not significantly increase in the ammonium acetate extractable ('proxy' of plant-available) fraction over 130 years. In addition, we compiled an overview of the global rate of mineral P fertilizer-derived U accumulation in agricultural sys-tems using existing literature (36 experimental trials, from 11 countries). The resulting dataset predicts an estimated mean U accumulation of 0.85 mu g U kg(-1) soil for an annual application of 1 kg P ha(-1) in the topsoil of agricultural sys-tems (0.26 mu g U kg(-1) per kg P ha(-1) for arable land and 1.34 mu g U kg(-1) per kg P ha(-1) for grassland). The annual U accumulation per applied kg P ha(-1) being 0.08 (Broadbalk) and 0.17 mu g U (Park Grass) corresponds to around one-third and one-eighth of the worldwide mean U accumulation for their respective agricultural systems, suggesting 'relatively' low U contents of the applied P fertilizers. Our study underscores that fertilizer-derived U accumulation is a persistent problem on the global scale, even if at different rates, and therewith suggests an evaluation of current regulatory limits and acceptable U input levels from P fertilization.

Automated summary

The application of phosphorus fertilizers leads to the accumulation of uranium in agricultural soils, which is a persistent and global problem. However, the proportion of fertilizer-derived uranium in the soil has not significantly increased over the past 130 years.