Tracing the fate of phosphorus fertilizer derived cadmium in soil-fertilizer-wheat systems using enriched stable isotope labeling.

Applying mineral phosphorus (P) fertilizers introduces a considerable input of the toxic heavy metal cadmium (Cd) into arable soils. This study investigates the fate of P fertilizer derived Cd (Cd) in soil-wheat systems using a novel combination of enriched stable Cd isotope mass balances, sequential extractions, and Bayesian isotope mixing models. We applied an enriched Cd labeled mineral P fertilizer to arable soils from two long-term field trials with distinct soil properties (a strongly acidic pH and a neutral pH) and distinct past mineral P fertilizer application rates. We then cultivated wheat in a pot trial on these two soils. In the neutral soil, Cd concentrations in the soil and the wheat increased with increasing past mineral P fertilizer application rates. This was not the case in the strongly acidic soil. Less than 2.3% of freshly applied Cd was taken up by the whole wheat plant. Most of the Cd remained in the soil and was predominantly (>95% of freshly applied Cd) partitioned into the easily mobilizable acetic acid soluble fraction (F1) and the potentially mobile reducible fraction (F2). Soil pH was the determining factor for the partitioning of Cd into F1, as revealed through a recovery of about 40% of freshly applied Cd in F1 in the neutral pH soil compared with about 60% in the strongly acidic soil. Isotope mixing models showed that F1 was the predominant source of Cd for wheat on both soils and that it contributed to over 80% of the Cd that was taken up by wheat. By tracing the fate of Cd in entire soil-plant systems using different isotope source tracing approaches, we show that the majority of Cd remains mobilizable and is potentially plant available in the subsequent crop cycle.