Cadmium losses in overland flow from an agricultural soil

Cadmium (Cd) transport in overland flow from agricultural soils is potentially important when trying to predict future soil Cd concentrations, but at present there is very little information on the magnitude of loss from this pathway. This study measured Cd concentrations and fluxes in overland flow from a catchment where cattle winter-grazed a forage crop (kale) (Brassica oleracea) in year one and measurements continued in year two when the catchment was returned to pasture and grazed by sheep. Flow-weighed mean concentrations (FWMC) of total, particulate and dissolved Cd in overland flow events from the forage crop were 0.49 µg L-1, 0.41 µg L-1 and 0.08 µg L-1 respectively. In contrast, no dissolved Cd was detected in overland flow from pasture, with a FWMC of 0.09 µg L-1. In line with the Cd concentrations, total Cd fluxes were greater from the forage crop (0.06 g Cd ha-1) than from pasture (0.04 g Cd ha-1). Cadmium losses in overland flow were relatively minor compared with those reported for other pathways such as plant uptake or subsurface flow. Further, compared to the amount of Cd that is currently added to soil in a maintenance application of phosphate fertiliser (30 kg P ha-1 yr-1), Cd losses in overland flow represented less than 1% of inputs. Measurement of Cd losses in overland flow should be undertaken at other sites with different soil types, topography and landuses to confirm the relatively low Cd losses found in this study, along with the distribution between dissolved and particulate fractions.