Dinitrogen Fixation by Soybeans in Alabama1

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Abstract
The contribution of symbiotic N2 fixation to total N and yield of soybeans [Glycine max (L.) Merr.] ranges widely among soils and cultural conditions. Utilization of nonnodulating soybean isolines and 15N‐dilution techniques has provided estimates of fixation in indeterminate cultivars on soils of relatively high N supply. Few estimates are available, however, for determinate soybeans on low N soils of the southeastern United States. This work compared the N assimilation of ‘Lee’ soybean with that of its nonnodulating isoline (N‐N Lee) at 10 locations in Alabama over an 8‐year period. Nitrogen accumulations by higher yielding cultivars were also determined. Dinitrogen fixation was estimated as the difference in the amount of N contained in N‐N Lee and Lee or other fixing cultivar. One approach considered N in harvested beans only while another approach involved total N in aboveground plant material at physiological maturity. Results showed that Lee soybeans grown in Alabama without irrigation but with rainfall sufficient for yields exceeding 1640 kg ha−1 fixed an average 172 kg ha−1 of N, while maximum yielding cultivars fixed 217 kg ha−1 of N in aboveground plant material. The estimate for midwestern soybeans of 40% of plant total N derived from the atmosphere is too low for Alabama soybeans, where 70% or more is obtained by fixation.

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