Speciation and phytoavailability of cadmium in selected surface soils of South Australia

A modified sequential extraction scheme was developed for partitioning the particulate-bound cadmium (Cd) into 9 fractions: exchangeable, carbonate-bound/specifically adsorbed, metal-fulvic acid-complex-bound, metal-humic acid-complex-bound, easily reducible metal oxide-bound, organic-bound, amorphous mineral colloid-bound, crystalline Fe oxide-bound, and detrital (bound to mineral lattices). Results on 11 surface soils showed that Cd in these soils was predominantly present in detrital form, bound to the mineral lattices, accounting for 15.8-61.9%, with an average of 33.4%, of the total Cd in the soils. The average relative abundance of Cd bound to the different particulate forms in the soils is in the order: detrital (0.077 mg/kg) > specifically adsorbed/carbonate-bound (0.066 mg/kg) > organic-bound (0.033 mg/ kg) > metal-fulvic acid-complex-bound (0.031 mg/ kg) > easily reducible metal oxide-bound (0.019 mg/ kg) > exchangeable (0.013 mg/ kg) > metal-humic acid-complex-bound (0.011 mg/ kg) > crystalline Fe oxide-bound (0.001 mg/ kg) amorphous mineral colloid-bound (0.001 mg/ kg). The phytoavailable Cd content was determined as Cd concentration in the shoot and leaf of durum wheat plants grown on the soils in a greenhouse study. Statistical treatment of the data showed that the exchangeable Cd (r = 0.735, P = 0.01) and the metal-fulvic acid-complex-bound Cd (r = 0.824, P = 0.002) correlated significantly with the plant-available Cd, compared with other species. The exchangeable and fulvic acid fraction of the metal-organic-complex-bound Cd contents, together, could explain 91.5% of the variation in plant-available Cd, determined as Cd concentration in leaf and stem of the durum wheat plants (r = 0.956, P = 0.0001). The significance of metal-fulvic acid complexes on Cd phytoavailability has not been reported so far and needs in-depth research in explaining the toxicity and food chain contamination of Cd in the environment.