Soybeans (Glycine max (L.) Merr.) have a high N requirement which is fulfilled by soil N uptake and N2-fixation. This study was concerned with the effects of past yield selection on N2-fixation in soybeans.
The soybean cultivars, ‘Lincoln’, ‘Shelby’, and ‘Williams’, which represent successive improvements in the ‘Lincoln’ germplasm, and a non-nodulating control were planted in a soil containing15N labelled organic matter. Two replications occurred on soil previously cropped to alfalfa and two on soil previously cropped to soybeans. Plants were harvested at five growth stages and leaf area, plant weight, total N, and atom percent15N were determined. Mature grain was harvested and yield components were also determined, as well as the total N and15N content.
Cultivar differences in total dry matter were only evident at physiological maturity, when Williams contained the greatest dry matter. Williams exhibited the longest period of seed formation and seed fill and also had the highest grain yield which resulted from a larger weight per seed.
The N content of the cultivars did not vary until physiological maturity when Williams contained the highest percent N. The quantity of N fixed at physiological maturity was highest for Williams and lowest for Lincoln. Fixed N contained in the harvested grain was greater for Williams than for the other two cultivars. The fraction of the total plant N derived from fixation was not greatly affected by cultivar and all cultivars acquired an average of 50% of their total N through N2-fixation.
Previous cropping history greatly affected the quantity of N fixed and the fraction of the total plant N derived from fixation. Soybeans following soybeans were more dependent upon N2-fixation than soybeans following alfalfa with the former deriving 65% of the total plant N from fixation and the latter only 32%. These soybean cultivars apparently utilized soil N first and then used N2-fixation to satisfy their N requirement.
The past selection for higher yield has resulted in soybean cultivars with improved capacities to fix atmospheric N2 and an improved ability to take up available soil N.