A new chemical reaction model created by ORNL incorporating iron, carbon and oxygen concentrations and soil acidity suggests that iron cycling can have a profound effect on whether carbon dioxide or methane is produced in repeatedly flooded Arctic soils, as described in the Journal of Geophysical Research: Biogeosciences. Iron can be used by soil microorganisms as a substitute for oxygen under waterlogged conditions, which produces carbon dioxide using a strategy that is typically thought to compete with methane production.

“Conventional wisdom says that if there’s more iron in the soil it should reduce the amount of methane that’s produced,” Sulman said. “But we found that’s not necessarily true. Under the right conditions we see a synergistic interaction where iron cycling changed the acidity of the soil in a way that also made it more advantageous for methane production.”

Flooding, drying of soils affect emissions

The scientists found that repeated flooding and drying of soils can be a key driver in permafrost greenhouse gas emissions by replenishing forms of iron that are gradually depleted by flood.

The models are based on data gleaned from incubation experiments with soils collected in Alaska’s permafrost as part of the Next-Generation Ecosystem Experiments Arctic, or NGEE Arctic. Results from those experiments highlighted the importance of iron cycling and led to the modeling work, in what is known as a ModEx paradigm, Sulman said.

ModEx, or the model-experiment approach, integrates observations and measurements from experiments with computer models of the same processes in an ongoing conversation that informs both the improvement of models and the design and interpretation of experiments. This loop ensures that models reflect the latest findings on critical systems, and the simulations in turn identify knowledge gaps and guide future experiments.

“Typically, soil research has focused on either dry or wet conditions, but not both,” said ORNL’s Elizabeth Herndon, whose soil and water sampling work in the Arctic provided data for the project. “The reality is that soil saturation can fluctuate a lot. You’ll have a rain event, and that results in oxygen-depleted conditions where microorganisms are essentially breathing iron to drive their metabolism. But when the soil dries, oxygen is reintroduced. In this paper, we wanted to explore what happens when you have these widely varying conditions.”