Ammonium Oxidation and Nitrate Reduction in Sediments of a Hypereutrophic Lake
- E. M. D'Angelo and
- K. R. Reddy
Internal N cycling processes in sediments and the overlying water column may contribute to the eutrophication of lake systems. One of the major mechanisms for N loss in these systems is through biological oxidation and reduction of N species in the aerobic and anaerobic sediment zones, coupled with exchange processes between these zones. These mechanisms were measured using flooded, intact sediment columns and batch incubations with bulk sediments collected from a hypereutrophic lake. In continuously stirred batch incubations with aerated sediment, NH+4 oxidation to NO−3 (nitrification) showed two-phase, zero-order kinetics. The rapid first phase of nitrification (0.36 mg N L−1 h−1) was due to the oxidation of NH+4 initially present in the sediment, while the slower second phase (0.15 mg N L−1 h−1) was limited by the rate of production of NH+4 during ammonification of organic N. Denitrification as determined by the C2H2-blockage technique was found to be limited by NO−3 availability. Under NO−3 nonlimiting conditions, the denitrification rate was 0.11 mg N L−1 h−1, but a fivefold decrease was measured at low NO−3 concentration (1 mg NO3 L−1). Denitrification was the major NO−3 reductive process in the surface 27-cm sediment depth. Assimilatory 15NO−3 reduction into the organic 15N fraction was also a significant NO−3 loss mechanism at the sediment surface. Dissimilatory 15NO−3 reduction to 15NH+4 became the dominant NO−3 reductive pathway at sediment depths >27 cm. Losses of up to 90% of the floodwater 15NH+4 or 15NO−3 was largely attributed to sequential nitrification-denitrification reactions.
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