Do soil cadmium concentrations decline after phosphate fertiliser application is stopped: A comparison of long-term pasture trials in New Zealand?

Abstract

Decreasing soil cadmium (Cd) is one method of removing Cd from the food chain. Phosphorus (P) fertilisers are a major source of Cd inputs into soil. Stopping P fertiliser should theoretically decrease Cd inputs and soil Cd accumulation, but there are few field data to show if this occurs. We examined three long-term grazed pasture trials in New Zealand (Ballantrae, Winchmore and Whatawhata) where P fertiliser had been applied (from 10 to 100\xa0kg P ha-1\xa0yr-1) for up to six years and then stopped for 10 to 26\xa0years. Stopping P fertiliser applications reduced soil Cd concentrations at Winchmore and Whatawhata where P had been applied at ≥34\xa0kg P ha-1\xa0yr-1. No reductions occurred below this rate nor at Ballantrae where only 10\xa0years post P-application data were available. Decreases were ascribed to moderate rainfall (1630\xa0mm at Whatawhata and 740\xa0mm rainfall plus 770\xa0mm irrigation at Winchmore) that enhanced Cd leaching and may have been aided at Winchmore by a decrease in soil pH over time (0.4\xa0units). However, because stopping P fertiliser inputs may quickly impair pasture production, additional strategies may be required to decrease soil Cd quickly.