Isotopic differences in cadmium between soil types and land uses

Abstract or Summary

Cadmium (Cd) isotope analysis is a potentially powerful tool for distinguishing Cd sources in contaminated systems, and considerable interest exists in New Zealand in differentiating Cd derived from fertilisers and Cd originating from other natural sources. This approach has recently been demonstrated at a long-term field trial where Cd isotope ratios (δ114/110Cd) of fertiliser and native sub-soil were distinct (separated by up to 0.5 ‰). But a key test of the application of isotope tracing to the problem of distinguishing sources more generally (e.g. in waterways, biota, etc) is that background Cd (e.g. in non-fertilused soils) should be isotopically distinct from phosphate fertilisers. Therefore we set out to determine the isotope ratio of Cd (δ114/110Cd) in topsoil from unfertilised (n=8), pastoral (n=6) and cropping (n=4) land-uses. The results showed that on average soils showed a hierarchy of increasingly positive δ114/110Cd values between unfertilised (0.10 ± 0.07 ‰), cropping (0.14 ± 0.07 ‰) and pasture (0.20 ± 0.07 ‰) soils, consistent with mixing between native (less positive δ114/110Cd) and pre-2000 phosphate fertiliser (more positive δ114/110Cd) sources. However, the potential for aeolian transport of Cd from the surrounding fertilised lands cannot be excluded as an additional factor perturbing the unfertilised δ114/110Cd results. Therefore, further experiments are needed to define a pre-human δ114/110Cd for New Zealand settings before Cd isotope tracing can be applied more generally to the New Zealand environment.