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  • APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Nov. 1983, p. 1084-1092 Vol. 46, No. 50099-2240/83/111084-09$02.00/0Copyright C 1983, American Society for Microbiology

    Sediment Nitrification, Denitrification, and Nitrous OxideProduction in a Deep Arctic Laket

    K. M. KLINGENSMITH AND V. ALEXANDER*Institute of Marine Science, University of Alaska, Fairbanks, Alaska 99701

    Received 22 February 1983/Accepted 19 July 1983

    We used a combination of 15N tracer methods and a C2H2 blockage technique todetermine the role of sediment nitrification and denitrification in a deep oligotro-phic arctic lake. Inorganic nitrogen concentrations ranged between 40 and 600nmol cm3, increasing with depth below the sediment-water interface. Nitrateconcentrations were at least 10 times lower, and nitrate was only detectable withinthe top 0 to 6 cm of sediment. Eh and pH profiles showed an oxidized surface zoneunderlain by more reduced conditions. The lake water never became anoxic.Sediment Eh values ranged from -7 to 484 mV, decreasing with depth, whereaspH ranged from 6.0 to 7.3, usually increasing with depth. The average nitrificationrate (49 ng of N *cm-3 * day-1) was similar to the average denitrification rate (44ng of N * cm-3 * day-1). In situ N20 production from nitrification and denitrifica-tion ranged from 0 to 25 ng of N* cm-3 * day-'. Denitrification appears to dependon the supply of nitrate by nitrification, such that the two processes are coupledfunctionally in this sediment system. However, the low rates result in only a smallnitrogen loss.

    Limnological studies of nitrogen cycle pro-cesses have generally emphasized nutrient con-centrations, inorganic nitrogen assimilation,and, more recently, nitrogen regeneration toammonia. Little attention has been devoted tothe rates of regeneration to nitrate or to the roleof denitrification as a nitrogen sink. However,recently there has been increased interest in therole of N20 in freshwater systems. N20 is anintermediate in both nitrification and denitrifica-tion, and the processes which control its produc-tion and consumption are currently of consider-able concern (15).

    In high-latitude lacustrine systems, previousnitrogen cycle work did not specifically addressnitrification (1; V. Dugdale, Ph.D. thesis, Uni-versity of Alaska, Fairbanks, 1965), althoughdenitrification was studied in two subarcticlakes, Smith Lake (10) and Ace Lake (R. C.Clasby, M.S. thesis, University of Alaska, Fair-banks, 1972). In these studies, 15N was used,and any nitrogen which was only reduced to theN20 stage was not included in the estimate (12).The only truly arctic aquatic data available aredenitrification rates for arctic pond sediment innorthern Alaska. The extremely low rates (0.17to 0.19 ng ofN * cm-3 * day- 1) found in this casemay have been an underestimate since, in thisenvironment, N20 may have been a significant

    tContribution no. 529, Institute of Marine Science, Univer-sity of Alaska, Fairbanks.

    component of the denitrification products andwas not included in the estimate (18).

    Toolik Lake, a deep arctic lake, was chosenfor our study as part of a larger, system-orientedproject (17). Of interest was the potential fornitrogen loss to the lake through coupled nitrifi-cation and denitrification.Lake sediments usually have a surface oxi-

    dized layer and become more reduced withsediment depth, so that nitrification and denitri-fication are spatially separated. Ammonium isproduced in sediments through the microbialoxidation of nitrogenous organic matter, and thiscan take place in both zones. Depending on theammonium gradient in the sediment, the ammo-nium can diffuse upwards into the water columnor downwards deeper into the sediment. In theoxic layer, the ammonium becomes oxidized tonitrite or nitrate by chemoautotrophic microbialprocesses (nitrification). These forms are moremobile and can diffuse upwards into the watercolumn and serve as a nutrient source or diffusedownwards into a more reduced zone and pro-vide the substrate for denitrification. Denitrifica-tion may also occur in anoxic microzones in thesurface sediment.N20 production can result from either denitri-

    fication or nitrification (3, 4, 7, 20, 27, 28) sinceit is an intermediate product in nitrification andan intermediate or, under some conditions, aterminal product of denitrification. To date,however, very few measurements of N20 pro-

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  • NITROGEN TRANSFORMATION IN A DEEP ARCTIC LAKE 1085

    Shoals from ,n Daerial photo

    Peak 5 Depth (m)

    500 m

    FIG. 1. Toolik Lake map showing depth contours and sampling sites (A).

    duction and consumption have been published.As a natural constituent of the atmosphere, N20concentrations range between 200 and 500 ppb(200 and 500 nI/liter, respectively), and its possi-ble role in the ozone layer of the earth hasprovided an impetus for research on the factorsregulating the global cycle. The increased use offertilizers presumably has increased N20 pro-duction through nitrification and denitrification.No previous information exists on the role ofnitrification and denitrification in an oligotrophicarctic lake in which the water column remainsoxygenated during all seasons. In this paper wepresent evidence that nitrification and denitrifi-cation occur simultaneously at low rates in thissystem.

    Toolik Lake is a large, deep oligotrophic lakeon the North Slope of Alaska, located at 68038'N and 149036' W. It lies in the north-facingfoothills of the Brooks Range within the trans-Alaska pipeline corridor. It is a kettle lake, withfive basins separated by rocky shoals and an

    area of 3.06 km2 at an elevation of 720 m. Theaverage depth is 7.45 m, and the maximum depthis 25.3 m. There is one major inlet and oneoutlet, but during spring runoff, as many as fivesmall inlet flows develop. Toolik Lake is ther-mally stratified during the summer, and icecovers the lake from October to mid-June. Ageneral description of the lake is given by Millerand Hobbie (17).

    Smith Lake, near Fairbanks, was also used inthis study to check the methodology for measur-ing denitrification. Located within the Universi-ty of Alaska's arboretum, this lake is known tohave very high denitrification rates in winterunder ice cover (10) after becoming anoxic bymid-February each year (1).

    MATERIALS AND METHODS

    Sediment cores were taken in the mouth of a smallbay (Fig. 1) under 10 m of water on five occasionsbetween 29 May and 15 September 1979 and threeoccasions in 1980. In August 1980, a transect was

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  • 1086 KLINGENSMITH AND ALEXANDER

    nmol N- cm 3

    O 200 400 6000-3 . . r

    3-6. May 29, 1979

    6-9 \

    C 9-12

    II 15-16,-1

    18-21 ik21-24

    3-6

    6.9

    9-12

    12-15

    15-18

    18621

    21-24

    nmol N - cm, 3

    nmol N - cm- 3

    400 600 800

    ItI s A.u>st 2,1979t4X,IX

    nmol N cm- 3

    o 200 400 600

    July 31, 1979

    '4

    nmol N -cm, 3

    FIG. 2. Toolik Lake sediment interstitial and exchangeable inorganic nitrogen during 1979. Interstitial NH4'(--0-- ), exchangeable NH4' (-*-), and interstitial N03 + N02- (-------) at 10 m.

    made with samplings at water depths of 2, 5, and 10 m.Permanent buoys were used to mark the samplinglocations during the ice-free period.The cores were obtained with a KB gravity corer

    (Wildlife Supply Co.) modified to fit a 3-in (7.62-cm)core tube and were immediately brought back to thefield laboratory to be sectioned for experiments, nutri-ent analysis and pH and Eh measurements. Sedimentcores were used for experiments and for analysis onlyif they were apparently undisturbed.

    Interstitial water was collected in situ from equili-brators (13) and from cores with Reeburgh (19) coresqueezers. Nitrate, nitrite, ammonium, and exchange-able ammonium determinations were made by themethod of Strickland and Parsons (25) in the fieldlaboratory. Exchangeable ammonium was extractedwith 2 M KCI for 24 h, a variation of the technique ofBlackburn (2). Organic nitrogen was determined afterignition at 750C. A Coleman nitrogen analyzer, whichuses Dumas combustion, was used for total nitrogendeterminations on dried sediment.

    Simultaneous Eh and pH measurements were madewith an Orion combination pH electrode and an OrionEh electrode. The electrodes were inserted at least 2cm beneath the surface of the core to the depth ofinterest in an effort to eliminate atmospheric contami-nation.

    Nitrification was measured by a 15N-NO3- dilutiontechnique (16) and by a 15N-NH4' tracer method.Sediment core samples were sectioned into 3-cm in-crements and placed in test tubes with 10 ml ofenriched labeled lake water containing 30 ,umol ofNH4' N * liter-' and 10 ,umol of N03- N - liter-'.Labeled nitrogen as NH4' or N03- was added as a30% enrichment of 15N. Samples were incubated for 0,24, and 48 h on the sediment surface and were filteredat the designated times. The filtrate was frozen untilfurther analysis. Both the dilution and the tracersamples were analyzed for 15N-N03- on a modifiedBendix Time-of-Flight model 17-210 mass spectrome-ter. The preparation for mass spectrometry was as

    follows. A 1-ml carrier, 5 ,umol of NO3- N * ml-', wasadded to the filtrates, and the nitrate was then reducedto nitrite with a Cu-Cd column (25). The Cu-Cdcolumn was ca. 95% efficient in nitrate reduction.Once nitrite was reduced to dinitrogen (16, 21), the15N/14N ratio was obtained, and the atom percent wascalculated with a precision of 0.01 atom %.The acetylene blockage technique (23, 29) was used

    to measure the rate of denitrification and N20 produc-tion. Core

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