Soil Microbes May Absorb Far More Methane Than Thought, Three-Model Study Finds

Microbes in soil may be soaking up much more methane than scientists had assumed, according to a new analysis published in the Journal of Geophysical Research: Biogeosciences. Methane is a greenhouse gas about 27 to 30 times more potent than carbon dioxide over 100 years, and certain soil microbes, called methanotrophs, biologically remove it from the atmosphere. Earlier estimates put the soil sink at an average of 28 to 35 gigatons of methane a year, and researchers suspected even that was too low.

The gap matters because two standard ways of estimating methane disagree. Bottom-up, process-based models, which track the underlying biological and chemical steps, tend to give higher emissions from wetlands and inland fresh waters than top-down atmospheric inversion models, which start from measured methane in the air and work backwards to its sources. A larger soil sink could help reconcile the two, bringing the bottom-up totals closer to what the atmosphere implies.

Youmi Oh and colleagues added a third approach, data-driven machine learning, and ran all three types of model in parallel to narrow the uncertainty. They also adjusted the microbial dynamics in the process-based model and included previously overlooked places and microbes. Both the process-based and machine-learning models produced similarly sized sinks, estimating that soil microbes take up 40 to 45 gigatons of methane globally each year, significantly more than older methods and larger than figures in assessments such as those of the Intergovernmental Panel on Climate Change (IPCC).

When the bigger soil sink was fed into top-down atmospheric calculations, it improved how well the models reproduced observed atmospheric methane and its carbon isotope makeup. The authors conclude that the microbial soil methane sink has been underestimated and that their three-model approach may improve global carbon-cycle modelling.