The rate of accumulation of cadmium and uranium in a long-term grazed pasture: implications for soil quality

Abstract

Cadmium (Cd) and uranium (U) are potentially toxic metals added to soil via phosphate fertiliser. Measured Cd and U concentrations in archived soils of the Winchmore long-term fertiliser indicated annual accumulation rates increased with the rate of superphosphate (SSP) applied, up to 5 µg Cd kg⁻¹ and 7 µg U kg⁻¹ in plots receiving 376 kg SSP ha⁻¹ y⁻¹. At the same rate of application, Cd was enriched in dryland compared to irrigated topsoil, while sampling to 30 cm showed that Cd, and less so U, moved down the profile in proportion to the rate of SSP applied and irrigation frequency. Using data in a mass balance suggested that soil Cd enrichment may have recently reached a plateau. The current rates of Cd and SSP application at Winchmore are unlikely to further enrich topsoil beyond the soil trigger value in the planned New Zealand Cd management strategy of 1 mg Cd kg⁻¹.

Keywords:

• leaching
• mass balance
• superphosphate
• Winchmore

Acknowledgements

Soil depth samples were provided by Bob Longhurst (AgResearch). The effort of Lynne Clucas (Lincoln University) in analysing the samples is greatly appreciated. This work was partly supported by the New Zealand Fertiliser Manufacturer's Research Association.