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 N 2 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 15 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 NO 3 − supply impairs cation uptake. © 2017 Society of Chemical Industry