Search

The difference method of measuring nitrogen fixation in soybeans ( Glycine max (L.) Merr.) relies on the assumption that nodulated and nonnodulated plants utilize equal amounts of soil nitrogen (N). The validity of this assumption has been challenged by reports of differences in root system size for nodulated and nondulated and nondulated genotypes. This field study, using a 15 N-tracer technique under conditions with reduced available soil N, was conducted to: (1) evaluate thee assumption that the nodulated and nonnodulated soybean genotypes utilize equal amounts of available soil N, (2) compare the root system mass in the nodulated and nonnodulated genotypes, and (3) determine the relationship between soil-N accumulation in the shoots and root dry-matter accumulation. In 1977 and 1978, one nonnodulated and six nodulated soybean genotypes were grown in replicated field plots labeled with 15 N on an Aqualfic Normudult (silt loam) soil. Plantts 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. Shoot and root dry-matter accumulation, seed yield (in 1977 only), total-N concentration, total-N accumulation, soil-derived N accumulation, and fixed-N accumulation were measured. Significant differences were present among genotypes both years for all variables except soil-derived N accumulation in the shoots. Shoot and root weights for the nonnodulated genotype could not be distinguished statistically from the nodulated genotype either year, although the seed yield for the nonoddulated was only 37% of the mean of yields for the nodulating genotype in 1977. Root dry weight was positively correlated with shoot dry weight ( r e = 0.90 for, 1977, r w = 0.86 for 1978), seed yield ( r w = 0.79 for 1977), shoot total-N accumulation ( r w = 0.82 for 1977, r w = 0.68for 1978), soil-derived N accumulation in the shoots ( r w = 0.71 for 1977), and accumulation of fixed-N in the shoots ( r w = 0.84 for 1977, r w = 0.85 for 1978). Results of this study support the assumption of equal utilization of soil-derived N by nodulated and nonnodulated soybean genotypes. The use of the difference method under conditions of low N fertility is appropriate for preliminary studies and in areas where 15 N-tracer techniques are not available.