Potential rates of nitrification and denitrification in an oligotrophic freshwater sediment system

Potential rates of nitrification and denitrification were measured in an oligotrophic sediment system. Nitrification potential was estimated using the CO oxidation technique, and potential denitrification was measured by the acetylene blockage technique. The sediments demonstrated both nitrifying and denitrifying activity. E_h, O₂, and organic C profiles showed two distinct types of sediment. One type was low in organic C, had high O₂ and E_h, and had rates of denitrification 1,000 times lower than the other which had high organic C, low O₂, and low E_h. Potential nitrification and denitrification rates were negatively correlated with E_h. This suggests that environmental heterogeneity in denitrifier and nitrifier populations in oligotrophic sediment systems may be assessed using E_h before sampling protocols for nitrification or denitrification rates are established. There was no correlation between denitrification and nitrification rates or between either of these processes and NH₄⁺ or NO₃^– concentrations. The maximum rate of denitrification was 0.969 nmole N cm^–3 hour^–1, and the maximum rate of nitrification was 23.6 nmole cm^–3 hour^–1, suggesting nitrification does not limit denitrification in these oligotrophic sediments. Some sediment cores had mean concentrations of 6.0 mg O₂/liter and still showed both nitrification and denitrification activity.