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Factors Influencing Nitrate Transformations in Sediments¹

ABSTRACT

The effects of environmental and procedural factors (e.g., aeration, size of sample, water/sediment ratio, nitrate concentration, glucose addition, temperature, and sterilization) on denitrification in lake and reservoir sediments were evaluated by incubating nitrate-amended natural sediments in the laboratory. The standard laboratory procedure adopted for denitrification studies involved the incubation of small samples of sediment (undried) after addition of a nitrate solution. Incubations were conducted at 15C in polypropylene centrifuge tubes. Denitrification in sediment-water samples was not enhanced by continuously purging samples with helium to create artificial anaerobic conditions. Experiments showed that denitrification rates in sediments could be satisfactorily measured using wet sediment (0.5 g solids) treated with 5 ml of 100 ppm nitrate-¹⁵N solution and incubated for 5 to 10 days. The small sample size facilitated sample preparation, extraction of inorganic forms of N and micro-Kjeldahl analysis of sediment.

Even though glucose additions enhanced denitrification by 25 to 30%, unamended sediments contained sufficient available organic-C for near maximal rates of denitrification. Denitrification in sediments increased with increasing temperature over the range of 5 to 23C. However, significant denitrification occurred at temperatures as low as 5C. Nitrate was not lost from sterilized sediment samples, therefore, the nitrogen transformations observed in unsterilized sediment samples were mediated by biological mechanisms.

The evaluation of factors affecting denitrification in sediment shows that the environmental conditions likely to exist in natural sediments are suitable for denitrification. Anaerobiosis and the high organic matter of most sediments create an ideal environment for microbial denitrification. The levels of nitrate in surface water may in part be controlled naturally by denitrification in sediments.

Key Words: denitrification • immobilization • eutrophication

¹ Journal Paper No. 5776, of the Indiana Agricultural Experiment Station. This work was supported in part by funds from the United States Department of Interior, Office of Water Resources Research.

² Research Assistant and Associate Professor of Agronomy, respectively.

Received for publication January 28, 1975.

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