The Tale of a Neglected Energy Source: Elevated Hydrogen Exposure Affects both Microbial Diversity and Function in Soil.

The enrichment of H-oxidizing bacteria (HOB) by H generated by nitrogen-fixing nodules has been shown to have a fertilization effect on several different crops. The benefit of HOB is attributed to their production of plant growth-promoting factors, yet their interactions with other members of soil microbial communities have received little attention. Here we report that the energy potential of H, when supplied to soil, alters ecological niche partitioning of bacteria and fungi, with multifaceted consequences for both generalist and specialist microbial functions. We used dynamic microcosms to expose soil to the typical atmospheric H mixing ratio (0.5 ppmv) permeating soils, as well as mixing ratios comparable to those found at the soil-nodule interface (10,000 ppmv). Elevated H exposure exerted direct effects on two HOB subpopulations distinguished by their affinity for H while enhancing community level carbon substrate utilization potential and lowering CH uptake activity in soil. We found that H triggered changes in the abundance of microorganisms that were reproducible yet inconsistent across soils at the taxonomic level and even among HOB. Overall, H exposure altered microbial process rates at an intensity that depends upon soil abiotic and biotic features. We argue that further examination of direct and indirect effects of H on soil microbial communities will lead to a better understanding of the H fertilization effect and soil biogeochemical processes. An innovative dynamic microcosm chamber system was used to demonstrate that H diffusing in soil triggers changes in the distribution of HOB and non-HOB. Although the response was uneven at the taxonomic level, an unexpected coordinated response of microbial functions was observed, including abatement of CH oxidation activity and stimulation of carbon turnover. Our work suggests that elevated H rewires soil biogeochemical structure through a combination of direct effects on the growth and persistence of HOB and indirect effects on a variety of microbial processes involving HOB and non-HOB.