Natural background and anthropogenic contributions of cadmium to New Zealand soils

The need to manage the potentially biotoxic metal-cadmium (Cd) in soil, relative to proposed limits, requires knowledge of how much Cd is present naturally under minimally disturbed conditions (MDC). Two data sets were collated, one of MDC soils sampled under native bush (n = 293) and another of different land uses (arable, dairy, drystock, horticulture, forestry, and urban; n = 1043) from which was estimated the background and anthropogenic contributions of Cd. The concentration of total Cd in MDC soils was strongly correlated to 11 different variables (e.g. pH, total carbon), but to total phosphorus most of all (r = 0.712, P < 0.001). Phosphorus concentration was used in an equation to show on average that background concentrations (up to a maximum of the 95th percentile of the MDC data set; 0.48 mg kg−1) accounted for about half of the Cd in the land use data set. The New Zealand fertiliser management strategy currently sets absolute limits – irrespective of soil type or background contributions. However, given that anthropogenic inputs of Cd have been shown to be more bioavailable to plants, our approach could be used to highlight and initiate further investigation of anthropogenic enrichment that is more soil specific.

► The concentration of Cd in reference soils was analysed and modelled.
► Natural background Cd accounted for nearly half of the Cd in different land uses.
► Soil limits should consider the contribution to total Cd made by natural Cd.