Cadmium in agricultural soil: The distribution and storage of soil cadmium as influenced by an agricultural mixed-downs hill-country landscape

Concern has been raised over the accumulation of cadmium (Cd) in soil, a by-product of phosphate (P) based fertilisers, because of long residence in soil and the potential for adverse effects on environmental and human health that arise with exposure to Cd. Research into Cd has been primarily focussed on the properties of Cd in the soil which influence soil Cd storage. Cd is not distributed evenly across the landscape and the interaction of landscape factors and farm management may influence this distribution which is an area of research that is less well understood. Therefore, the purpose of the research is to assess, through the use of a theoretical model, whether the current understanding of Cd behaviour in the farming landscape is sufficient for the prediction of soil Cd, as influenced by landscape and management factors. As a subsidiary study, the impact of liming on Cd storage is assessed. Soil samples were taken from a mixed-downs hill-country farm with varied topography and known fertiliser history. This site selection allowed the comparison of Cd and soil properties in a range of different farming landscapes, with emphasis placed on predictions made from the theoretical model. The concentration of soil Cd found in this study does not exceed New Zealand soil standards or guidelines; however concentrations are elevated beyond the control which indicates anthropogenic influence. Evaluation of results with regard to the theoretical model revealed and confirmed the link between P application and Cd in soil. Lower Cd levels were found in slopes when compared to other terrain types which agrees with theoretical predictions. This suggests that a combination of landscape and farm management practices is responsible for this result. However, the effect of land-cover on soil Cd was less apparent and did not support predictions. With regard to the combined interactions of terrain and P application states, ridge tops that had high P application statistically had the greatest likelihood of contributing to high Cd. Preferential grazing of ridge tops and potential runoff processes were considered as potential reasons for this result. Contrary to expectations from theory, higher concentrations of exchangeable Cd were found in soils that had received high lime application. This finding could not be explained in the context of current theory, but could be significant in terms of how liming is used as a tool for remediation of Cd