Soil cadmium and New Zealand dairy farms: Impact of whole-farm contaminant variability on environmental management

Two long-term dairy farms with different phosphorus (P) fertiliser application history in the North and South Island of New Zealand were studied to understand factors influencing the spatial distribution of soil cadmium (Cd). Intensive soil sampling within each farm showed that soil Cd concentrations (0-150 mm depth) were much greater and more variable in the Waikato (North Island) farm (mean: 1.04 mg kg−1, range: 0.48-1.64 mg kg−1) than the Canterbury (South Island) farm (mean: 0.34 mg kg−1, range: 0.15-0.64 mg kg−1). A significant (P < 0.001) relationship between total soil Cd and total P indicated the overriding influence of P fertiliser application history on soil Cd accumulation. However, within land management units (LMUs) of common P fertiliser history, soil type had a significant (P < 0.001) influence on soil Cd concentration. Slope class had no influence (P = 0.491) on soil Cd concentration within the range of 0−15°. There was no clear, consistent relationship between soil Cd and land-based effluent disposal on either farm. Given the large spatial variability in soil Cd concentration that was evident within the two farms of this study, it is recommended that sampling strategies used for the assessment of soil Cd (e.g. under environmental regulation/management frameworks) need to account for variation in land use, P fertilisation history, and soil type. Sampling individual paddocks based on predominant soil type will also allow areas to be identified where there is heightened risk of plant Cd accumulation.