Trace metal accumulation in agricultural soils from mineral phosphate fertiliser applications in European long-term field trials

Mineral phosphate (P) fertilisers are the main suspected sources of uranium (U) and cadmium (Cd) input to agricultural soils. This study was set up to survey the general long-term impact of P fertilisers on concentrations of Cd, U and other trace metals in European soils. A total of 218 soil samples was collected from 15 long-term (3-78 years, median 18 years) P trials at 11 locations with a pairwise comparison of topsoil composition between the fertilised and corresponding unfertilised soils. Aqua regia soil extraction detected the effects of fertilisation more sensitively than the HF (real total) soil extraction. Statistically significant differences in aqua regia soluble metals due to fertiliser application were detected more frequently for U (10 of 15 trials) than for Cd (4 of 15 trials). The concentrations of U and Cd in soil linearly increased with cumulative applied P across all soils and sites; a total addition of 1 ton P ha(-1) increased the mean topsoil (23 cm depth) concentrations by 0.11 mg U kg(-1) soil (0.09-0.12 mg U kg(-1), 95% CI) and 0.03 (0.02-0.04) mg Cd kg(-1) soil. These results correspond with mass balance predictions (1 ton P ha(-1); +0.15 mg U kg(-1) soil and +0.02 mg Cd kg(-1) soil) based on previously determined average trace element concentrations in European fertilisers. Data thus suggests that, on average among trials, losses of Cd from soil are undetectable. In contrast, about 30% of the theoretical U input from fertilisers is lost in the same trials. This study provides data to better evaluate the modelled trace metal accumulations in soil and to evaluate the prevailing metal limits in mineral fertilisers. Highlights U and Cd accumulation in agricultural soils correlates with cumulative mineral P-fertiliser dose. Accumulation of fertiliser-derived U is more often detectable compared to Cd. Due to its accumulation potential, U accumulation should be monitored. Aqua regia soluble concentrations better detect metal accumulation in soil than real total concentrations.

Automated summary

Cumulative use of mineral P fertilisers in agricultural soils leads to linear increase in concentrations of uranium (U) and cadmium (Cd), with U accumulation being more detectable. The study provides data for better evaluation of trace metal accumulations in soil and prevailing metal limits in mineral fertilisers.