Dose-response relationships between environmentally-relevant H2 concentrations and the biological sinks of H2, CH4 and CO in soil

Abstract

Local H2 accumulations can be found in soil, especially within legume crop agroecosystems, where H2 is an obligate by-product of nitrogen fixation. Recent investigations show that diffusive fluxes of H2 act as additional energy inputs shaping microbial community structure and function in soil. The goal of this study is thus to define dose-response relationships between H2 exposure and soil microbial community dynamics. Community structure and trace gases (i.e. H2, CH4 and CO) oxidation activities were investigated following soil incubation to environmentally-relevant H2 mixing ratios. Despite no evidence of an alteration of microbial diversity, coordinated dose-response relationships between trace gases oxidation rates and H2 exposure were recorded. Measured H2 oxidation rates were implemented into a theoretical framework modeling H2 decay as a function of distance from H2-emitting point sources. Theoretical H2 concentration profiles and dose-response relationships between H2 concentration and trace gases oxidation rates were integrated to predict the impact of H2 on microbial community functioning. While most H2 is oxidized within 1 cm of H2 point sources, trace gases oxidation is predicted to be altered within a 10 cm radius. High-affinity CH4 and CO oxidation capacities dropped by up to 78% and 84% along H2 concentration gradients, respectively. Theoretical distances from H2-emitting point sources required to reactivate 50% of maximal CH4 oxidation rate were 0.3 cm in farmland soil and 0.2 cm in poplar soil. A longer distance was required to reactivate 50% of CO oxidation rate, i.e. 1.5 and 2.7 cm in farmland and poplar soils, respectively. Loss of CH4 oxidation potential observed under elevated H2 exposure was correlated with a gain of low-affinity H2 oxidation activity, while a substrate inhibition of high affinity H2 oxidation rate was paralleled with a decreasing trend of CO oxidation activity. In addition to shedding light on potential interactions between H2, CH4 and CO biogeochemical processes in soil, these novel findings provide evidence that H2 supports metabolic and energetic flexibility in microorganisms supplying a variety of ecosystem services.