Fate of dung-applied copper in a British grassland soil

Copper is commonly used as a fodder amendment in cattle nutrition, but little is known about the relation between dung turnover and environmental fate of Cu. We devised a Cu budget for a common British grassland soil (Typic Epiaquept) for the initial 70 days after the application of 5.33 kg m−2 of dry dung mass in 0.045 m2 patches. Three replicated treatments: (i) no dung control, (ii) Cu-amended dung (3.2 g Cu m−2), and (iii) unamended dung (i.e. no extra Cu in the animal feed, 1.0 g Cu m−2) were established. Total Cu concentrations were determined in the 0–1, 1–5, and 5–10 cm depth layers 14, 28, 42, and 70 days after dung application. We also determined Cu, pH, and dissolved organic C (DOC) concentrations in the soil solution at 30 cm depth. Initially, Cu concentrations ranged between 24 and 26 mg kg−1 at all depths in all plots and did not change in the control during 70 days at any depth. The recovery of the applied Cu in the remaining dung, soil, and leachate after 70 days was 5%, 10%, and 2% for the Cu-dung treatment and 18%, 8%, and 2% for the unamended dung treatment leaving >70% of the dung-derived Cu unaccounted for in both dung treatments. In soil, applied Cu was sorbed preferentially to aggregate surfaces where we detected 30 mg Cu kg−1 whilst the aggregate interior contained 26 mg Cu kg−1. However, the differences between aggregate interior and exterior disappeared 28 days after application. The Cu concentrations in soil solution were closely related with DOC concentrations but not with pH values. The percentage of dung-derived Cu and C lost by leaching were almost identical. Our results suggest that (i) dung-applied Cu is mainly distributed and lost laterally, (ii) aggregate turnover in a grassland soil may occur within one month, and (iii) the fate of applied dung Cu is quantitatively linked to the fate of dung C.