Impact of rhizobial inoculation and reduced N supply on biomass production and biological N 2 fixation in common bean grown hydroponically.

BACKGROUND: Testing rhizobial inoculation of common bean (Phaseolus vulgaris L.) in hydroponics enables accurate quantification of biological N2 fixation (BNF) and provides information about the potential of reducing inorganic N fertilizer use. In view of this background, common bean grown on pumice was inoculated with Rhizobium tropici CIAT899 (Rt) and supplied with either full-N (total nitrogen 11.2 mmol L-1 ), 1/3 of full-N or N-free nutrient solution (NS). BNF was quantified at the early pod-filling stage using the15 N natural abundance method.

RESULTS: Full-N supply to Rt-inoculated plants resulted in markedly smaller nodules than less- or zero-N supply, and no BNF. Rt inoculation of full-N-treated plants did not increase biomass and pod yield compared with non-inoculation. Restriction (1/3 of full-N) or omission of inorganic N resulted in successful nodulation and BNF (54.3 and 49.2 kg N ha-1 , corresponding to 58 and 100% of total plant N content respectively) but suppressed dry shoot biomass from 191.7 (full-N, +Rt) to 107.4 and 43.2 g per plant respectively. Nutrient cation uptake was reduced when inorganic N supply was less or omitted.

CONCLUSION: Rt inoculation of hydroponic bean provides no advantage when full-N NS is supplied, while 1/3 of full-N or N-free NS suppresses plant biomass and yield, partly because the restricted NO3 - supply impairs cation uptake. © 2017 Society of Chemical Industry.