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

Abstract

A method is described for the determination of the net and total rates of NH 4 + production and NH 4 + incorporation at different depths in an anoxic marine sediment. 15 N-NH 4 + was added to the sediment NH 4 + pool, and the 15 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 of 15 N-NH 4 + dilution. A simple microdiffusion of NH 3 was followed by an emission spectrometry analysis of 15 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 of 15 N-NH 4 + between interstitial and exchangeable NH 4 + 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 NH 4 + production was 274 (standard deviation, 31) nmol cm −3 day −1 , and the mean total rate of NH 4 + 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.