The potential biofertilization effect of H is accompanied by a modest impact on the composition of microbial communities in the rhizosphere of common vetch.

BACKGROUND

N-fixing nodules release molecular hydrogen (H) in the rhizosphere of legumes. The process activates H-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 on the soil microbial community structure and the population density of HOB has received little attention. The aim of the current study is to evaluate how the legacy effect of HOB, previously activated in soil microcosms exposed to elevated H concentrations (eH), affects biomass production yield of common vetch (), the abundance of HOB, and the composition of the rhizosphere microbiome.

METHODS

Contrasting soil samples displaying more than 60% difference in H oxidation activity were used as growth substrate for vetch. Soil microbial community composition and diversity were examined by bacterial 16S rRNA polymerase chain reaction (PCR) amplicon sequencing, and dry weight (DW) of the above- and below-ground biomass of vetch was analyzed to assess the impact of HOB enrichment on plant growth. The population density of high-affinity HOB was estimated by using the droplet digital polymerase chain reaction (ddPCR) technique to target the gene, encoding for the large subunit of group 1H/5 [NiFe]-hydrogenase.

RESULTS

The abundance of HOB possessing group 1H/5 [NiFe]-hydrogenase was indistinguishable between the treatments, indicating that soil nutrient content (inorganic and organic carbon) and the energy potential of H were insufficient to support their growth. spp. and spp. were favored by eH exposure at the activation stage, but their response was lost after vetch growth. The root biomass and the root/shoot ratio were reduced in soil conditioned with eH compared to control soil exposed to ambient H, suggesting that the plant growth-promotion activity of HOB reduces root proliferation for nutrient prospection. These results provide new experimental evidence suggesting the biofertilization effect of H is not universal and requires specific conditions that are yet to be identified.