Impact of rhizobial inoculation and reduced N supply on biomass production and biological N2 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 the 15N 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.