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Published online 1 September 1982
Published in Agron J 74:799-804 (1982)
© 1982 American Society of Agronomy
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Field Comparison of the Nitrogen-15 and Difference Methods of Measuring Nitrogen Fixation1

Helen-Jean Talbott, W. J. Kenworthy and J. O. Legg2

Further investigation of the methods of evaluating N2 fixation under field conditions is essential, since the accuracy and reproducibility of the method of measurement have profound effects on the results of an experiment; and, opinions differ concerning which method provides the best estimates. The amount of N2, fixed and the percentage of total plant N from biologically fixed N in field-grown soybeans [Glycine max (L.) Merr.] measured by the difference method were compared with corresponding estimates from a 15N-tracer technique in which 15N-labeled soil organic matter is used as the tracer material. In 1977 and 1978, six N2-fixing soybean genotypes were grown in rows alternating with rows of a nonnodulating genotype (Clark rj, rj,) in replicated field plots on a Mattapex silt loam (fine-silty, mixed, mesic Aquic Hapludult) where the 15N was incorporated into the soil organic fraction. The difference method compared total N contents of the N2-fixing and adjacent nonnodulating plants. The 15N method determined the dilution in the N2-fixing plants of soil-derived 15N by symbiotically-fixed N. Plants were sampled at 49, 70, and 91 days after planting and at full maturity in 1977, and at 62 and 112 days after planting in 1978. Sources of variation for the estimates were identified as the total N accumulation in the nonnodulating genotype for the difference method, and the 15N concentration in the nonnodulating genotype for the 15N method. Estimates from the difference and 15N methods for the amount of total N fixed were highly correlated (r = 0.89 for 1977, r = 0.92 for 1978). The percentage of total N fixed vaned between the two methods (r = 0.32 for 1977, r = 0.69 for 1978) which was attributed to spatial variation of available soil N. The 15N method provided more accurate estimates for the percentage of total N fixed because the 15N concentration was less variable than the total N accumulation in the nonnodulating genotype.

Key Words: Glycine mar (L.) Merrill • 15N-dilution • Nonnodulated soybeans

1 Scientific article No. A3037, Contribution No. 6100 of the Maryland Agric. Exp. Stn., Dep. of Agronomy, College Park, MD 20742; ahd USDA, ARS, Beltsville, MD 20705. Part of a thesis submitted by the senior author in partial fulfillment of the requirements for the M.S. Degree

2 Physical science technician, USDA, ARS, and former graduate student at the Univ. of Maryland; associate professor of agronomy, Univ. of Maryland; and forme: soil scientist with USDA, ARS, currently adjunct professor, Univ. of Arkansas; respectively.

Received for publication September 8, 1981.




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