Trace metal enrichment in sugarcane soils due to the long-term application of fertilisers, North Queensland, Australia: geochemical and Pb, Sr, and U isotopic compositions

摘要

This study aimed to determine whether >80 years of fertiliser application has led to recognisable changes in the trace metal (Cd, Cu, Mo, Ni, Pb, Sr, Th, U, Zn) chemistry of topsoils (0–0.10 m) from sugarcane land, northern Queensland, Australia. The metal concentrations of commercial nitrogen (N) and potassium (K) fertilisers currently used in northern Queensland were generally lower than those of phosphate fertilisers and fertiliser blends. Composite topsoil samples (0–0.10 m depth) taken from canelands had higher median Cd, Mo, Pb, Sr, Th, U, and Zn concentrations than topsoils from forested areas of the catchment. Niobium, Ta, and Ti were confirmed as refractory immobile elements and used as reference elements for the evaluation of trace metal enrichments. Bivariate plots of trace metal/immobile element ratios verified that Cd, Mo, Pb, Sr, Th, U, and Zn are enriched in sugarcane soils compared with background forest soils. Isotopic ratios for Pb, Sr, and U highlight that fertilisers, cane soils, and forest soils have isotopically distinct compositions. Phosphate fertilisers currently used in the agricultural industry possess the most radiogenic 87Sr/86Sr, 234U/238U, 207Pb/206Pb, and 208Pb/206Pb ratios. Background forest soils have the highest 87Sr/86Sr, 207Pb/206Pb, and 208Pb/206Pb and lowest 234U/238U ratios. By contrast, cane soils exhibit 207Pb/206Pb and 208Pb/206Pb ratios that appear on a mixing line between the isotopically distinct background soils and phosphate fertilisers. Also, cane soils possess 234U/238U ratios similar to phosphate fertilisers. Thus, the application of phosphate fertilisers to canelands has resulted in higher Cd, Mo, Pb, Sr, Th, U, and Zn concentrations and more radiogenic Pb, Sr, and U isotope ratios in cane soils. Trace metal ratios and the Pb, Sr, and U isotopic composition of topsoils and fertilisers are useful tools to recognise fertiliser-derived trace metals in agricultural landscapes.