Revisiting and Updating the Effect of Phosphorus Fertilisers on Cadmium Accumulation in European Soils

Keywords: phosphate rock, soil cadmium, mass balance, soil quality

Cadmium (Cd) occurs naturally as an impurity in rock phosphate at concentrations of 1-200 mg Cd (kg P₂O₅)^-1 and is present in commercial phosphate (P) fertilisers. The application of P fertilisers is an important source of annual addition of Cd to the agricultural soils of Europe.

In 2002, a proposal was made to limit Cd concentrations in EU fertilisers based on a mass balance approach, in which the balance is made between Cd inputs (atmospheric deposition, P fertiliser use, lime, sludge or manure application) and the Cd outputs (crop offtake and leaching). In this 2002 study, a Cd concentration in P fertilisers was calculated that would not lead to a net accumulation of Cd in agricultural soils of EU-15 over 100 years, assuming all mass balance parameters remained constant. The conclusion of this study was that no net accumulation is expected when P fertilisers containing maximally 20 mg Cd (kg P₂O₅)^-1 are applied to the soils. This has led to a proposal of setting Cd limits for P fertilisers in Europe.

The objectives of the present study were (i) to update that mass balance with actual input and output parameters and data applicable to the

EU-27+Norway and (ii) to calculate scenarios of long-term change in soil Cd depending on the Cd concentration in the P fertilisers. The update is warranted on the basis that emissions of Cd have decreased since the assessment of 2002, the use of P-fertilisers has decreased and there are now better tools to estimate Cd leaching from soil.

Future long-term changes (100 years) in soil Cd concentrations were calculated for four fertiliser Cd concentrations (20, 40, 60 and 80 mg Cd (kg P₂O₅)^-1) and for 2160 different scenarios covering the range of conditions encountered in the EU, i.e. P fertiliser use, soil properties (pH, organic carbon content), climatic conditions affecting leaching, type of crop and atmospheric deposition (zero to worst case); and for the European average values. The following conclusions could be drawn:

· At the highest Cd concentration studied, 80 mg Cd ( P₂O₅)^-1, soil Cd is predicted to remain rather constant.

· At 60 mg Cd ( P₂O₅)^-1, soil Cd is predicted to change by -7% in 100 years, i.e. a net decrease.

· At 40 mg Cd (kP₂O₅)^-1, these values are -14% and at 20 mg Cd (kg P₂O₅)^-1 they are -20%.

· The current average EU Cd mass balance is negative compared to positive balances estimated in 2002.

Prof. Erik Smolders, KU Leuven, Division Soil and Water Management, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium.

30 pages, 10 figures, 10 tables, 37 references.