Nodulation Response to Different Soil Mineral Nitrogen Concentrations in Modern and Obsolete Soybean Cultivars.

Soybeans derive most of their nitrogen from the atmosphere through symbiotic nitrogen fixation. Selecting for enhanced nitrogen fixation based on nodulation has had limited success in soybean breeding. Many laboratory and field experiments have documented inhibition of soybean nodulation by applied nitrogen. Elevated residual soil nitrogen levels originating from high fertility practices during the corn growing season, in addition to the prevalence of corn-soybean crop rotations raise questions of possible deleterious effects on soybean nodulation. To assess changes in nitrogen fixation as a result of breeding in an environment with changing mineral nitrogen concentrations, a greenhouse study was initiated to observe a gain or loss of nodulation in soybean cultivars released over time. Ten genotypes released between 1930 and 2000 were grown in the greenhouse and watered with ammonium nitrate solutions with nitrogen concentrations of 0, 2.13, 4.25, 6.38, or 8.56 mM. Chlorophyll content was measured twice over the duration of the experiment, and seven weeks after planting the plants were harvested. Nodule numbers, size, and fresh and dry weights were quantified as well as root biomass. Corresponding stem and leaf dry matter was determined and analyzed for total nitrogen and ureides. In all genotypes, nitrogen treatments had a significant effect on nodule characteristics. Cultivar x nitrogen treatment interactions will be discussed.