Nitrogen Metabolism in Lakes I. Measurement of Nitrogen Fixation With N151

A method for measuring the rates of nitrogen fixation in samples of lake water using N 15 as a tracer is described, and a detailed description of glassware required is provided. The nitrogen present in the water is removed by flushing with Hc‐O mixture and then replaced by the labeled nitrogen. After incubation and treatment, the isotope ratio in the organic material is determined on a mass spectrometer. Analyses of lake water and (NH 4 ) SO 4 as isotope ratio blanks are described and a résumé of statistical data from these analyses is presented.