The phytoavailability of cadmium in some New Zealand soils

In New Zealand, phosphate fertiliser use has resulted in an accumulation of cadmium (Cd) in some pasture soils, and has led to an elevation in Cd concentrations in pasture species growing on these soils. While there is an abundance of literature on the soil chemistry of Cd, there is shortage of data dealing with low soil Cd concentrations that are relevant to New Zealand soils, typically < 1.5 µg Cd g⁻¹ soil. The purpose of this study was to determine some of the factors that affect the phytoavailability of Cd in soils at the low Cd concentrations relevant to New Zealand conditions, and to ultimately identify management strategies to minimise Cd uptake into plants. In a study using six soils to investigate the effect of soil pH on sorption/desorption of both native and added Cd, it was revealed that with increasing pH (between pH 4.9 - pH 6.2) there was a substantial reduction in the concentration of both native and added Cd desorbed from soils using 0.01 M Ca(NO₃)₂. Conversely it was found that there was an increase in the amount of added Cd sorbed as pH increased from 4.9 to 6.2. Soil organic carbon was also identified as playing an important role in controlling sorption and desorption of Cd. Regression analysis using a larger set of soils (29), indicated that soil pH, organic carbon and total soil Cd were the most dominant soil variables controlling Cd solubility. In addition, the same three soil parameters were important for governing desorption of native Cd into soil solution. Soil pH and organic carbon were important in the sorption Cd, while CEC along with soil pH were important in controlling added Cd desorption. In addition, increasing soil pH was shown in a glasshouse study to significantly decrease plant Cd concentrations, in five different plant species. There was evidence of immobilisation of Cd in soils as a result of increased contact time between the soil and added Cd. Results from an incubation study, where Cd was added to the soil as Cd(NO₃)₂, revealed that for four very different soils, there was a decrease in the concentration of Cd desorbed into solution using 0.01 M Ca(N0₃)₂with an increasing contact period of up to 70 days prior to desorption. Additional evidence of Cd immobilisation with contact time was derived from a long-term superphosphate trial, where Cd had been added to soil, fertiliser application ceased and the same soil re-sampled 21 years later. Results from the trial indicated that during this 21 year hiatus, the proportion of Cd in soluble forms, as determined by a chemical fractionation, decreased, while there was an increase in Cd in residual fractions. A study of the same soil which had received annual superphosphate fertiliser application for 44 years, revealed through chemical fractionation, that there was an increase in Cd associated with the exchangeable and organic fractions with time, however there was still a substantial proportion of Cd in non-available residual forms. Similarly, for 12 topsoil samples in a separate study, while there was a wide range in the concentrations of Cd associated with individual soil fractions, the greatest concentration of Cd was associated with the organic and residual fractions, with substantial proportions of Cd added as a result of fertiliser application present in the residual, and presumably non-phytoavailable, fraction. In a glasshouse study, the concentrations of Cd in a number of vegetable, pasture, and cereal species was investigated. Plant Cd concentrations varied greatly between different plant species and soil types. All plants were within the maximum residue limit (MRL) for the Cd content of all foodstuffs intended for human consumption. In an evaluation of soil extractants that are commonly used to predict plant Cd concentrations, it was found that a unbuffered or neutral extractant such as CaCl₂, NH₄OAc, or Ca(N0₃)₂ which was sensitive to soil pH was a suitable predictor of plant Cd concentrations. An investigation into the rate of Cd accumulation in soil from a long-term (44 yr) superphosphate fertiliser trial was carried out. Results indicated that there had been a significant accumulation of Cd in soil that has been subject to long term superphosphate fertiliser application relative to the control plot. On the high fertiliser treatment (376 kg superphosphate ha⁻¹ yr⁻¹), Cd was estimated to have accumulated at a rate of 7.8 g ha⁻¹ yr⁻¹. There was also evidence of movement of Cd down the soil profile in this irrigated soil. This study has identified several management options available for the control of Cd phytoavailability in soils. The maintenance of a soil pH within a target range of 5.8-6.0 will not only increase sorption of Cd but will decrease desorption of Cd and reduce Cd solubility. The adoption of farming practices which maintain adequate organic matter levels in the soil should be encouraged. The selection of P fertilisers with low Cd concentrations is to be promoted to reduce soil Cd concentrations. Finally, as a management strategy, a 'hands off' approach may be suitable given that there seems to be fixation of Cd in soil with increased residence time.