Root Architectural Traits Enhancing Topsoil Foraging Increase Symbiotic Nitrogen Fixation in Common Bean (Phaseolus vulgaris L.).

Low P availability is a primary constraint to the production of common bean in Africa and Latin America. Genetic variation for root traits that enhance topsoil foraging is being used to generate new bean genotypes with greater P acquisition efficiency. Studies were conducted at Lichinga (Mozambique) in 2008 and at Ukulima (South Africa) in 2010 to test the hypothesis that P-efficient common bean genotypes have better nodulation and better N₂ fixation than conventional genotypes. Ten genotypes of two P-efficiency categories were grown with two phosphorus levels and inoculation/liming combinations. P-efficient genotypes had significantly greater nodulation efficacy than P-inefficient genotypes. Under low P, inoculation increased the nodule number of P-efficient genotypes by 34 to 71%. For P-inefficient genotypes, inoculation increased nodule number by between 20 to 46%. Nodule activity of P-efficient genotypes were consistent in both years and were between 50.6% and 92.5%, a rate at least 30% greater than for P-inefficient genotypes. For the P-inefficient genotypes the nodule activity varied between 27.5 and 33.4%. For liming and inoculation treatments the contrast of responses between the two categories of genotypes was much larger. Shoot dry weight was generally greater for P-efficient genotypes (13.5 to 9.1 g plant^-1) compared to 9.5 to 7.5 g plant^-1 for the P-inefficient genotypes under low P. For P-inefficient genotypes there was no significant difference between inoculated and non-inoculated treatments under low phosphorus in either year. Under low P inoculated treatment the genotype L88-57 fixed the equivalent of 39 kg of urea ha^-1 followed by G19833 and L88-14 fixing N equivalent to 32 kg of urea, and the smallest amount was fixed by the genotype DOR 364 with the equivalent of 1.6 Kg of urea (46%) per ha. These results suggest that the introduction of P-efficient genotypes would enhance soil N status in low input agroecosystems.