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Simultaneous Determination of Nitrification and Nitrate Reduction in Sediment-Water Columns by Nitrate-15 Dilution¹

ABSTRACT

The magnitude of N loss from nitrification-denitrification reactions in sediment-water columns of a freshwater lake of Louisiana Gulf Coast drainage basin receiving elevated N loading from agricultural runoff was determining using ¹⁵NO₃ dilution. Isotope dilution using added labeled NO₃ allowed for the simultaneous determination of nitrification and NO₃ reduction in the sediment-water column. The contribution of internal NH₄ pools in bottom sediment to NO₃ content in water column through diffusion and nitrification was measured. Nitrate reduction averaged 134 µmol m^–2 h^–1 over a 50-d incubation period in the sediment-water columns. Nitrification averaged 90 µmol m^–2 h^–1, approximately 70% of the denitrification rate. Nitrification and ammonium assimilation were also determined in lake water using ¹⁵N-labeled NH₄ as a tracer. The nitrification rate determined in the water (0.9–1.4 mmol N m^–3 d^–1) was less than the rates measured in the sediment-water columns (1.4–3.3 mmol N m^–3 d^–1) using ¹⁵NO₃ dilution. The results presented and isotope dilution technique used are applicable for use in understanding the fate of N in sediment-water systems of other aquatic environments.

Key Words: water quality • mineralization • denitrification • eutrophication • Louisiana • N loading

¹ Contribution from Laboratory for Wetland Soils and Sediments, Center for Wetland Resources, Louisiana State Univ., Baton Rouge, LA 70803-7511.

² Assistant Professors, Laboratory for Wetland Soils and Sediments, Louisiana State Univ., Baton Rouge, LA 70803-7511.

Received for publication October 24, 1986.