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Field Comparison of the Nitrogen‐15 and Difference Methods of Measuring Nitrogen Fixation 1
Author(s) -
Talbott HelenJean,
Kenworthy W. J.,
Legg J. O.
Publication year - 1982
Publication title -
agronomy journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.752
H-Index - 131
eISSN - 1435-0645
pISSN - 0002-1962
DOI - 10.2134/agronj1982.00021962007400050008x
Subject(s) - loam , sowing , nitrogen , chemistry , mathematics , field experiment , significant difference , agronomy , horticulture , zoology , soil water , biology , ecology , statistics , organic chemistry
Further investigation of the methods of evaluating N 2 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 N 2 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 15 N‐tracer technique in which 15 N‐labeled soil organic matter is used as the tracer material. In 1977 and 1978, six N 2 ‐fixing soybean genotypes were grown in rows alternating with rows of a nonnodulating genotype (Clark rj 1 rj 1 ) in replicated field plots on a Mattapex silt loam (fine‐silty, mixed, mesic Aquic Hapludult) where the 15 N was incorporated into the soil organic fraction. The difference method compared total N contents of the N 2 ‐fixing and adjacent nonnodulating plants. The 15 N method determined the dilution in the N 2 ‐fixing plants of soil‐derived 15 N 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 15 N concentration in the nonnodulating genotype for the 15 N method. Estimates from the difference and 15 N 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 varied 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 15 N method provided more accurate estimates for the percentage of total N fixed because the 15 N concentration was less variable than the total N accumulation in the nonnodulating genotype.