The potential biofertilization effect of H 2 is accompanied by a modest impact on the composition of microbial communities in the rhizosphere of common vetch
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N 2 -fixing nodules releases molecular hydrogen (H 2 ) in the rhizosphere of legumes. The process activates H 2 -oxidizing bacteria (HOB) in soil, leading to multiple effects on biogeochemical processes and a potential biofertilization effect. The legacy effect of the energy potential of H 2 on soil microbial community structure and the population density of HOB has received little attention. The aim of this study is to evaluate how the legacy effect of HOB, previously activated in soil microcosms exposed to elevated H 2 concentration (eH 2 ), affects biomass production yield of common vetch ( Vicia sativa ) and the abundance of HOB and the composition of microbiome in the rhizosphere. Contrasting soil displaying more than 60% difference in H 2 oxidation activity were used as growth substrate for vetch. The abundance of HOB was indistinguishable between the treatments and two phylotypes, namely Aeromicrobium spp. and Ramlibacter spp., were favored by eH 2 exposure at the activation stage. Their response was transient as no legacy effect was noticed after vetch growth. The root biomass and the root/shoot ratio were reduced in soil conditioned with eH 2 . These results provide new experimental evidence suggesting the biofertilization effect of H 2 is not universal and requires specific conditions that remain yet to be identified.
