Organic carbon metabolism is a main determinant of hydrogen demand and dynamics in anaerobic soils.

Hydrogen (H) is a crucial electron donor for many processes in the environment including nitrate-, sulfate- and, iron-reduction, homoacetogenesis, and methanogenesis, and is a major determinant of microbial competition and metabolic pathways in groundwater, sediments, and soils. Despite the importance of H for many microbial processes in the environment, the total H consuming capacity (or H demand) of soils is generally unknown. Using soil microcosms with added H, the aims of this study were 1) to measure the H demand of geochemically diverse soils and 2) to define the processes leading to this demand. Study results documented a large range of H demand in soil (0.034-1.2 millielectron equivalents H g soil). The measured H demand greatly exceeded the theoretical demand predicted based on measured concentrations of common electron acceptors initially present in a library of 15 soils. While methanogenesis accounted for the largest fraction of H demand, humic acid reduction and acetogenesis were also significant contributing H-consuming processes. Much of the H demand could be attributed to CO produced during incubation from fermentation and/or acetoclastic methanogenesis. The soil initial total organic carbon showed the strongest correlation to H demand. Besides external additions, H was likely generated or cycled in the microcosms. Apart from fermentative H production, carboxylate elongation to produce C4-C7 fatty acids may have accounted for additional H production in these soils. Many of these processes, especially the organic carbon contribution is underestimated in microbial models for H consumption in natural soil ecosystems or during bioremediation of contaminants in soils.