Humic Acids Influence Microbial Respiration and Methanogenesis in Wetland Soils.

Methanogenesis in wetland soils contributes 40% of global methane emissions to Earth’s atmosphere, making it an important microbial process that governs climate. Even though methanogens compete poorly with other heterotrophic microbes for organic carbon (i.e. electrons), they dominate freshwater wetland soils because competing microorganisms can be limited by the supply of electron accepting compounds such as Fe(III), SO₄^2- or humic substances. Thus, methane production is ultimately controlled by the supply of both electron donating compounds (organic carbon) and electron accepting compounds (inorganic or organic). Humic substances are a dominate constituent of wetland soils that can act both as electron donors and electron acceptors in anaerobic microbial respiration. Yet, their role in microbial respiration is largely unknown. Here we demonstrate, for the first time, that humic acids extracted from a variety of wetland soils alter CO₂ and CH₄ production in anaerobic lab incubations. Humic acids extracted from a variety of wetland soils behaved differently, with some serving as electron donors and others serving as electron acceptors. Our results suggest that soil-derived humic substances may be an important, and currently unexplored, control of anaerobic decomposition in wetland soils.