An assessment of the effect of contact time on cadmium phytoavailability in a pasture soil

The length of time cadmium (Cd) is in contact with the soil has been recognised as a factor affecting phytoavailability, but the extent of this process is currently poorly understood. This study used isotopic dilution techniques (E and L values) to determine the effect of contact time on Cd phytoavailability from soil collected from a long term phosphorus (P) fertiliser trial. Cadmium phytoavailability was determined in soil that was last fertilised with soluble Cd from P fertiliser 17 years prior to sampling (residual plots), and soil that received annual applications of P fertiliser until sampling (continuous plots). It was found that both E values and L values increased with P fertiliser (viz Cd) inputs and were significantly related to each other (r2 = 0.82 P < 0.005). There was however no significant difference (P < 0.05) in the percentage of total Cd that was phytoavailable calculated using E values (E %) between the continuous (mean 51%) and the residual plots (mean 51%). There was also no significant difference (P < 0.05) in the percentage of total soil Cd that was phytoavailable calculated using L values (L%) between the continuous (mean 77%) and residual plots (mean 87%). These results suggest that despite Cd being in contact with the soil for 17 years, there was no difference in the size of the phytoavailable Cd pool compared to recent Cd inputs.