Biological nitrogen fixation by irrigated dry bean (Phaseolus vulgaris L.) genotypes

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Dry bean (Phaseolus vulgaris L.) is usually considered to be poor at biological nitrogen fixation (BNF), but large variations in this trait have been observed among bean genotypes. We evaluated 16 bean genotypes for N2 fixation ability in four N treatments: (i) uninoculated in low-N soil (30 kg N ha−1), (ii) inoculated with commercial Rhizobium leguminosarum bv. phaseoli inoculant Nitrastik-D® in low-N soil, (iii) inoculated with commercial R. leguminosarum bv. phaseoli inoculant Nodulator® in low-N soil, and (iv) uninoculated in high-N soil (100 kg N ha−1). There were differences between genotypes in all the plant parameters that were measured, but only nodulation was affected by N treatment. The 100 kg N ha−1 treatment suppressed nodulation. Seven genotypes nodulated well with either inoculant, two genotypes nodulated better with Nitrastik-D than with Nodulator, three nodulated better with Nodulator than with Nitrastik-D, and four nodulated poorly with either inoculant. Cultivars AC Redbond, Island, and Resolute, all currently commercially grown, did not fix much N2 at flowering (4–8 kg N ha−1) or maturity (19–34 kg N ha−1). By contrast, germplasm lines PI 136692 (red bean), GH-196 (pinto bean), and LEF2RB (carioca bean) had high BNF capability at flowering (10–11 kg N ha−1) and especially at maturity (60–72 kg N ha−1), in addition to high seed yield (2778–2897 kg ha−1), indicating their superior ability to support both of these economically important traits throughout plant growth. These three genotypes would be valuable to breeders for the genetic improvement of BNF in dry bean cultivars.