Isotope Tracing of Long-Term Cadmium Fluxes in an Agricultural Soil

Globally widespread phosphate fertilizer applications have resulted iri long-term increases in the concentration of cadmium (Cd) in soils. The accumulation of this biotoxic, and bioaccuniulative metal presents problems for the management of soil-plant-animal systems, because the magnitude and direction of removal fluxes (e.g., crop 'uptake, leaching) have been difficult to estimate. Here, Cd isotopic compositions (delta(CD)-C-114/110) of archived fertilizer and soil samples from a 66 year-long, agricultural field trial' in Winchmore, New Zealand, were used to constrain the Cd soil mass balance between 1959 and 2015 AD; informing future soil Cd accumulation trajectories. The isotopic partitioning of soil cd- sources in thiS system was aided by a change in phosphate, source rocks in 1998 AD, and a corresponding shift in fertilizer isotope composition. The.dominant influence of mixing between isotopically distinct Cd end-members was confirmed by a Bayesian modeling approach. Furthermore, isotope mass balance modeling revealed that Cd removal,processes most likely increased in Magnitude substantially between 2000 and 2015 AD, implying an increase,in Cd bioaccumulation and/or leaching over that interval. Natural-abundance stable, isotopes are introduced here as a powerful tool for-tracing the fate. of-Cd in agritultural soils, and potentially the wider environment. [GRAPHICS]