Subsurface cadmium loss from a stony soil – effect of cow urine application

Cadmium (Cd) losses in subsurface flow from stony soils that have received cow urine are potentially important, but currently poorly understood. This study investigated Cd loss from a stony soil under a winter dairy-grazed forage crop that was grazed either conventionally (24 hr) or with restricted grazing (6 hr). This provided an opportunity to test the hypothesis that urine inputs could increase Cd concentrations in drainage. It was thought this would be a result of cow urine either (i) enhancing dissolved organic carbon (DOC) concentrations via an increase in soil pH that results in the formation of soluble Cd-organic carbon complexes, or (ii) greater inputs of chloride (Cl) via cow urine, promoting the formation of soluble Cd-Cl complexes. Results showed that Cd concentrations in subsurface flow were generally low, with a spike above the water quality guidelines for a month after the 24 hr grazing event. Cadmium fluxes in drainage were on average 0.30 g Cd ha-1yr-1, in line with previous estimates for agricultural soils, equating to about 5% of that added in a maintenance application (30 kg P ha-1) of phosphate fertiliser. The mean Cd concentration in drainage in the 24 hr grazed plots was significantly higher (P < 0.05) than the 6 hr plots. No increase in DOC concentrations between treatments was found. However, Cl concentrations in drainage were significantly higher (P < 0.001) from the 24 hr than the 6 hr grazed treatment plots, and positively correlated with Cd concentrations; we suggest this is the reason for the greater Cd concentrations measured. This may mean that previous estimates of subsurface Cd losses from grazed agricultural systems that haven’t included urine inputs may have potentially underestimated subsurface Cd losses.