A bstract. The methodology, characteristics and application of the sensitive C,H.,-C2H4 assay for N2 fixation by nitrogenase preparations and bacterial cultures in the laboratory and by legumes and free-living bacteria in situ is presented in this comprehensive report. This assay is based on the N9ase-catalyzed reduction of C2Ho to C2H4, gas chromatographic isolation of C2H2 and C9H4, and quantitative measurement with a H,-fiame analyzer. As little as 1 ,u,lmole C..H4 can be detected, providing a sensitivity 103-fold greater than is possible with '-5N analysis.A simple, rapid and effective procedure utilizing syringe-type assay chambers is described for the analysis of C,H2-reducing activity in the field. To meet the imminent crisis in the world food supply (38) it is imperative that the resources of this planet be mobilized as rapidly and effectively as possible. Basic to such mobilization is a knowledge of the magnitude of the dynamic processes in the biosphere which affect the availability of nitrogen, the one element most often limiting in the production of foodstuffs (37

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