AMMONIUM OXIDATION AND NITRATE REDUCTION IN SEDIMENTS OF A HYPEREUTROPHIC LAKE

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, NH4+ oxidation to NO3- (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 NH4+ 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 NH4+ during ammonification of organic N. Denitrification as determined by the C2H2-blockage technique was found to be limited by NO3- availability. Under NO3- non-limiting conditions, the denitrification rate was 0.11 mg N L-1 h-1, but a fivefold decrease was measured at low NO3- concentration (I mg NO3 L-1). Denitrification was the major NO3- reductive process in the surface 27-cm sediment depth. Assimilatory (NO3-)-N-15 reduction into the organic N-15 fraction was also a significant NO3- loss mechanism at the sediment surface. Dissimilatory (NO3-)-N-15 reduction to (NH4+)-N-15 became the dominant NO3- reductive pathway at sediment depths >27 cm. Losses of up to 90% of the floodwater (NH4+)-N-15 or (NO3-)-N-15 was largely attributed to sequential nitrification-denitrification reactions.