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, NHJ oxidation to NOj (nitrification) showed twophase, zero-order kinetics. The rapid first phase of nitrification (0.36 mg N L-' h-) was due to the oxidation of NHJ initially present in the sediment, while the slower second phase (0.15 mg N L ' far) was limited by the rate of production of NHJ during ammoniflcation of organic N. Denitriflcation as determined by the C2H2-blockage technique was found to be limited by NOj availability. Under NOj nonlimiting conditions, the denitriflcation rate was 0.11 mg N L~' h-, but a fivefold decrease was measured at low NOj concentration (1 mg NO, L'). Denitriflcation was the major NOj reductive process in the surface 27-cm sediment depth. Assimilatory NOj reduction into the organic N fraction was also a significant NOj loss mechanism at the sediment surface. Dissimilatory NOj reduction to NH J became the dominant NOj reductive pathway at sediment depths >27 cm. Losses of up to 90% of the floodwater NH} or NO j was largely attributed to sequential nitrification-denitrification reactions. I ADDITION to anthropogenic inputs of N, bottom sediments in shallow lakes can act as an important source of N to the water column during (i) wind-driven sediment resuspension, (ii) diffusive flux of N in response to concentration gradients in the sediment, and (iii) bioturbation at the sediment-water interface. One of the major mechanisms of N loss in an aquatic system is through oxidation of NHJ" in the water column and in the aerobic sediment layer and reduction of NOf in the anaerobic zone of sediments (Reddy and Patrick, 1984; Seitzinger, 1988; Kemp et al., 1990). In the absence of external inputs, nitrification is the only mechanism by which NOf is added to the sediments. Nitrification may only occur in the water column or in the aerobic sediment-water interface. Once formed, NOf may readily diffuse into sediments in response to the concentration gradient established as a result of NOf reduction. Once NOf is in the sediments, physical, chemical, and biological conditions determine which of several NOf reduction process predominates. Denitrifying organisms predominate when anoxia is temporary, since their utilization of NOf (instead of O2) is a secondary process (Tiedje, 1982). On the other hand, organisms responsible for DNRA predominate in Cand electron-rich environments (Eh < -200 mV) (Buresh and Patrick, 1981; Tiedje et al., 1982). Assimilatory NOf reduction into Institute of Food and Agricultural Sciences, Soil and Water Science Dep., 106 Newell Hall, Univ. of Florida, Gainesville, FL 32607. Florida Agric. Exp. Stn. Journal Series no. R-02737. Received 25 Sept. 1992. * Corresponding author. Published in Soil Sci. Soc. Am. J. 57:1156-1163 (1993). organic N pool is repressed by large amounts of NHJ" , which often characterizes anaerobic lake sediments. Numerous field and laboratory studies have confirmed that these NOf reduction processes occur in a variety of aquatic environments, including estuary sediments (Nedwell, 1975; Nishio et al., 1982), marine sediments (Kaspar, 1982; Seitzinger and Nixon, 1985; Sorensen, 1978), and drainage ponds and lake sediments (Chan and Knowles, 1979; Chen et al., 1972a; Kaspar, 1985; Keeney et al., 1971). The N transformation processes are coupled reactions, in which the rate of one reaction (e.g., nitrification) may regulate the rate of the others (e.g., NOf reduction). Furthermore, the extent of the N biochemical processes in one ecosystem are different from that in another, so processes must be evaluated on an individual ecosystem basis. In shallow lakes, hydrodynamic processes can result in resuspension of bottom sediments into water column (Simon, 1988). During these events NH^ can be readily nitrified, and the NOf formed can be assimilated by algae and diffuse into anaerobic sediments, where it can undergo denitriflcation (Chen et al., 1972a,b). Nitrification, denitrification, DNRA, and the assimilation of NH.J^ and NOf in subtropical hypereutrophic lakes has yet to be quantified. The objectives of this study were to determine: (i) the rate of nitrification of NH^ in the aerobic zone of the sediment-water interface, (ii) the partitioning between denitrification DNRA, and ANR as a function of sediment depth, (iii) the maximum rate of denitrification in the sediment, and (iv) the influence of coupled processes of nitrification and NOf reduction on the overall N loss. MATERIALS AND METHODS Study Site and Sampling Lake Apopka is a shallow (mean depth = 1.7 m; area = 12 500 ha), hypereutrophic lake located northwest of Orlando in central Florida (chlorophyll a = 89 ± 6 ;u,g L~' [Newman, 1991]; total P = 0.22 ± 0.07 mg L-; TKN = 4.92 ± 0.85 mg L.-, [Reddy and Graetz, 1990]). Agricultural drainage water pumped from surrounding farms into the lake during the last 40 yr has caused progressive deterioration of the water quality of the lake (Reddy and Graetz, 1990). More than 90% of the lake sediment consists of a deep, unconsolidated flocculent layer (consisting of dead algal cells) underlain by a more consolidated layer of peat. A series of experiments was conducted using intact sediment columns and bulk sediment samples obtained from a single site that represents a major portion (90%) of the lake. Sediment cores (7.5-cm i.d.) used for the experiments were obtained to a depth of 40 to 50 cm with a piston corer (Fisher et al., 1992) in the spring and summer of 1989. The cores were allowed to equilibrate at room temperature (25 °C) before being used in the laboratory experiments. Abbreviations: DNRA, dissimilatory NO3reduction to NH4t ANR, assimilatory NOj reduction; TKN, total Kjehldahl nitrogen; TOC, total organic carbon; DEA, denitrification enzyme activity.