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 acc umulation and changes in forms of Cd in a soil that has been subjected to l ong-term superphosphate fertiliser application. Results indicate that there had been a significant accumulation of Cd in the soil during the past 44 y ears. 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). D uring the course of the trial, there was an increase in the proportion of C d associated with exchangeable and soil organic matter fractions on the fer tilised plots, which was related to a corresponding increase in soil organi c carbon levels. Results also indicate that although there was a large prop ortion 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 resid ence 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 decreas e in total soil Cd concentrations. There was also evidence of movement of C d down the soil profile in this irrigated soil. The implications of these r esults for Cd phytoavailability are discussed.