Soybean growth and seed yield (SY)
are closely related to nitrogen (N) which is provided by soil N sources,
fertilizer, and N2 fixation. In order to achieve yield potential,
soybeans must maintain high seed N concentrations while sustaining high photosynthesis
rates. We summarized differences in N uptake and N2 fixation by
soybean as affected by environments, genotypes or management, based on data published
in scientific journals from 1966 to 2004 and experiments conducted at the University of Nebraska-Lincoln. Most frequent protein
content ranged from 37 to 42% and N harvest index averaged 0.8. The slope of
the linear regression between SY and N uptake was 11 kg of SY per kg of N uptake
(r2 = 0.86, N = 412; P<0.01). Most frequent values of N2
fixation ranged from 61 to 151 kg N ha-1(36 to 61% of total N uptake), with
an overall range of 0 to 337 kg ha-1. Nitrogen fixation increased as
N uptake rose, reaching a maximum at 300-350 kg ha-1. The strength
of the relationship between SY and fixed N decreased when N fertilizer was
applied. An inverse non-linear relationship between N fertilizer rate and N
fixation was observed if fertilizer was applied at planting in the top layer of
soil. However, experiments where N was placed deeper or applied in reproductive
stages did not follow this trend. Significative response to N fertilizer was
recorded in lower-yielding environments (below 3500 kg ha-1) in
which the crop had constraints for growth, as well as in higher-yielding
environments, probably due to a higher N demand. Seed yield above the 75% percentile
(3320 kg ha-1), had 74% and 81% higher N uptake and N fixation. The
lack of data above this value, made difficult the prediction of crop behavior
and the role of N2 fixation in high-yielding soybeans.