Nitrogen Fixation by Soybeans: Seasonal and Cultivar Effects, and Comparison of Estimates1

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Abstract
Nitrogen fixation was estimated by acetylene reduction assays for 21 soybean [Glycine max(L)Merr.] cultivars from five maturity groups. Some cultivars were studied for 2 or 3 years. Fixation by four cultivars was also estimated by the isotope dilution method and by comparing N accumulation to that of a nonnodulating cultivar. Nitrogen fixation was related to maturity group. Late maturing cultivars fixed more N2 than early maturing cultivars. Within maturity groups there was little variation in nitrogen fixing activity. During a season when N2 fixation was low, there was little agreement among three methods of measuring fixation. When available soil N was artifically reduced by amendment with carbohydrate, N2 fixation was higher and there was good correlation among the techniques used. The acetylene reduction assay gave lower values than the other two methods.