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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.15 N-NH4 + was added to the sediment NH4 + pool, and the15 N 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 of15 N-NH4 + dilution. A simple microdiffusion of NH3 was followed by an emission spectrometry analysis of15 N 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 of15 N-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 17C 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 &lt;1% of these values in the 12- to 14-cm fractions.

applied and environmental microbiology

Method for Measuring Rates of NH 4 + Turnover in Anoxic Marine Sediments, Using a 15 N-NH 4 + Dilution Technique