Method for Measuring Rates of NH4+ Turnover in Anoxic Marine Sediments, Using a 15N-NH4+ Dilution Technique |
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| Authors: | T. Henry Blackburn |
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| Abstract: | A method is described for the determination of the net and total rates of NH4+ production and NH4+ incorporation at different depths in an anoxic marine sediment. 15N-NH4+ was added to the sediment NH4+ pool, and the 15N content was assayed after 0, 2, and 5 days of incubation. The pool size changed during incubation; this change in pool size is incorporated into a model which predicts the dynamics of 15N-NH4+ dilution. A simple microdiffusion of NH3 was followed by an emission spectrometry analysis of 15N content. This procedure avoided all problems of cross-contamination. The model was tested and rates were measured in four sediment cores, at seven different depths. The high correlation coefficients (mean, 0.96 for the 0- to 2-, 2- to 4-, 4- to 6-, and 6- to 8-cm sediment fractions) indicated that the model was correct and that the measured rates were valid. The immediate distribution of 15N-NH4+ between interstitial and exchangeable NH4+ pools indicated that it was the combined pool that was turning over. In the 0- to 2-cm fraction at 17°C the net rate of NH4+ production was 274 (standard deviation, 31) nmol cm−3 day−1, and the mean total rate of NH4+ production was 309 (standard deviation, 39) nmol cm−3 day−1; both rates decreased to <1% of these values in the 12- to 14-cm fractions. |
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