The role of environmental and management factors in the accumulation and plant bioavailability of cadmium in New Zealand agricultural soils : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Cadmium (Cd) accumulation in agricultural soils is a global issue, because its transfer to edible parts of common crop plants can pose a risk to food security. Soil Cd concentrations in NZ agricultural systems have been elevated due to the application of Cd-rich phosphorus fertilisers. The aim of this work was to investigate options for managing the potential Cd risk in NZ agricultural soils. To assess the main environmental factors determining Cd uptake into plants in the NZ soil environment, more than 500 soils and paired plant (edible parts) samples from around 100 cropping sites where lettuce, spinach, onion, potato and wheat were grown were collected. The results showed that mean Cd concentrations in lettuce, wheat, onion and potato were much lower than the maximum level of Cd in the Food Standard of Australia and New Zealand (FSANZ ML) of 0.1 mg kg-1 (fresh weight, FW). The mean Cd concentration in baby spinach and bunching spinach was 0.08 mg kg-1, with plant Cd concentrations at some sites approaching or exceeding this current NZ food standard. Around 7% of wheat grain samples also exceeded 0.1 mg kg-1 (FW). The effects of plant types and cultivars on plant Cd concentration were significant but varied between growing sites (regions) in this study. Multivariate regression analysis found that soil Cd concentration, pH and region could be used to estimate Cd concentrations in onions, while soil Cd concentration and carbon content could predict Cd concentration in bunching spinach. However, no significant relationships between plant Cd concentration and soil properties were observed in wheat grain and potato tuber. At a national scale, Cd concentrations in spinach and onion positively correlated with soil (pseudo) total Cd concentrations. Potato and wheat grown in Canterbury had a larger potential to accumulate Cd, although soil Cd concentrations in Canterbury were lower than in other regions. To investigate the relationship between soil Cd bioavailability and environmental factors in more detail, 147 paired soil and plant samples were randomly selected from the national survey for a more targeted investigation. Different methods were used to test the bioavailability of the Cd (pseudo-total and porewater concentrations, 0.05 M Ca(NO3)2-extraction and diffusive gradients in thin-films, DGT). Information on a variety of soil and climatic variables were also collected. The ability of these methods to predict the Cd concentrations in the various plant samples were then compared. The bioavailability testing showed that the predictive capability of these four methods varied between plants, with no single test providing an adequate prediction for all four species. Multivariate regression analysis showed that, once certain soil and climatic variables were accounted for, Ca(NO3)2 extractions could provide a satisfactory prediction of Cd uptake by onions and spinach, while the Cd accumulation in potato tuber and wheat grain were affected by various environmental factors, including: soil variables, fertiliser status, climatic condition and plant Zn concentration. The results of the bioavailability tests indicated that various environmental factors can combine to determine the uptake of Cd by different crop species and should be considered when estimating the risks posed by Cd in agricultural soils. Field trial experiments were conducted to evaluate the effects of lime (and sulphur) and municipal compost and their interactions on Cd uptake by potato and wheat. Three potato trials and one wheat trial were established. Cd concentrations in soil Ca(NO3)2 extractions and the edible plant parts were measured. The results showed that the application of lime and compost influenced Ca(NO3)2 extractable Cd concentration significantly in four trials. However, the changes in soil pH and organic carbon concentration by amendment addition did not achieve significant effects on Cd concentration in potato. Liming significantly increased Cd concentration in wheat grain. The variability of Cd concentrations in plant edible parts between potato and wheat across sites highlighted that the effects of lime and compost on soil Cd immobilisation were site dependent. Finally, a modelling exercise (SCM) was used to evaluate the effects of soil types, farming practices, irrigation levels, P application rate and Cd concentration in fertiliser on soil Cd accumulation. The SCM model was validated using soil monitoring Cd concentrations at the Winchmore long-term irrigation and fertiliser trials. The estimated soil Cd concentrations from the SCM model agreed closely with the topsoil measured Cd concentrations between 1952 and 2003. The SCM simulations suggested that fertiliser Cd concentration played an important role in soil Cd enrichment. The SCM model revealed that organic soils have the largest potential to accumulate Cd. In the next 25 years, soil Cd concentration in the Waikato may experience a slight increase with over 50% of dairy farms where soil concentrations would be likely to reach 1.0 mg Cd kg-1 soil, while no obvious tendency of soil Cd concentrations to increase is predicted in Canterbury, if the choice and rate of fertiliser products is allowed for when following the TFMS. This body of work highlights the risk in using soil variables as guidelines to manage the Cd accumulation in plants under field conditions. It is concluded that Cd accumulation in plants is soil and plant factors dependent. Also, Cd concentration may continue to increase in some organic C-rich soils and soils with intensive use of P fertilizers.