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

Trace elements can have a wide variety of effects on microbial populations and their function in the aquatic environment. However, specific impacts on chemical and biological processes are often difficult to unravel, due to the wide variety of chemical species involved and interactions between different elemental cycles. A replicated mesocosm experiment was used to test the effect of increasing copper concentrations, i.e., from 6 mg kg‾¹ to 30 and 120 mg kg‾¹, on nitrogen cycling in a freshwater sediment under laboratory conditions. Nitrous oxide emissions from the treated sediments were measured over three consecutive 24 h periods. This was followed by measurements of iron, manganese, copper and mineral nitrogen species (nitrate and ammonium) mobilisation in the sediments using the diffusive gradients in thin films (DGT) and diffusive equilibria in thin films (DET) techniques and sequential extractions. Increasing copper concentrations are shown to have resulted in significantly reduced nitrate formation near the sediment–water interface and increased nitrous oxide emissions from the sediment overall. The concomitant mobilisation and sequestration of iron with ammonium in the sediment with the highest Cu treatment strongly imply links between the biogeochemical cycles of the two elements. Modest Cu contamination was shown to affect the nitrogen cycle in the tested freshwater sediment, which suggests that even relatively small loads of the metal in fresh watercourses can exert an influence on nutrient loads and greenhouse gas emissions from these environments

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

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