The effect of long-term phosphatic fertiliser applications on the amounts and forms of cadmium in soils under pasture in New Zealand

This paper presents the results of an investigation into the rate of Cd accumulation and changes in forms of Cd in a soil that has been subjected to long-term superphosphate fertiliser application. Results indicate that there had been a significant accumulation of Cd in the soil during the past 44 years. 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). During the course of the trial, there was an increase in the proportion of Cd associated with exchangeable and soil organic matter fractions on the fertilised plots, which was related to a corresponding increase in soil organic carbon levels. Results also indicate that although there was a large proportion of added Cd associated with the organic fraction, the concentration of applied Cd occurring in the residual fraction was also substantial (i.e > 25% of added Cd). In addition, an investigation into the effects of residence time of Cd in the soil indicated that there was a redistribution of Cd into less soluble forms with time (i.e. residual Cd), along with a decrease in total soil Cd concentrations. There was also evidence of movement of Cd down the soil profile in this irrigated soil. The implications of these results for Cd phytoavailability are discussed.