Copper contamination affects the biogeochemical cycling of nitrogen in freshwater sediment mesocosms

Abstract

The effects of trace elements on microbial populations and function can be wide-ranging and difficult to disentangle, hence reports of specific impacts on processes in elemental biogeochemical cycles are rare. This study used a replicated laboratory mesocosm experiment that simulated a flowing watercourse to test the effect of increasing copper concentrations from 6 mg/kg to 30 and 120 mg/kg on nitrogen cycling in a freshwater sediment. After a seven-week equilibration period, the release of nitrous oxide from the treated sediments was measured over three consecutive 24 h periods. This was followed by measurements of copper (Cu), iron, manganese and nitrogen species (nitrate and ammonium) mobilisation in the sediments using diffusive gradients in thin films (DGT), diffusive equilibrium in thin-films (DET) and sequential extractions. Copper is shown to have resulted in significantly reduced nitrate formation near the sediment-water interface and increased nitrous oxide emissions. The concomitant mobilisation and sequestration of iron with ammonium in the sediment with the highest Cu treatment strongly implies links between the biogeochemical cycles of the two elements. This research concludes that modest Cu contamination can affect the nitrogen cycle in freshwater sediments, which may have further implications on water quality and greenhouse gas emissions from freshwater environments.