Soil researchers quantify an underappreciated factor in carbon release to the atmosphere

Soil plays a critical role in global carbon cycling, in part because soil organic matter stores three times more carbon than the atmosphere. Now biogeochemist Marco Keiluweit at the University of Massachusetts at Amherst and colleagues elsewhere for the first time provide evidence that anaerobic microsites play a much larger role in stabilizing carbon in soils than previously thought.

Further, current models used to predict the release of climate-active CO2 from soils fail to account for these microscopic, oxygen-free zones present in many upland soils, they say.

"Without recognizing the importance of anaerobic microsites in stabilising soil carbon in soils, models are likely to underestimate the vulnerability of the soil carbon reservoir to disturbance induced by climate or land use change," write first author Keiluweit and colleagues at Stanford, Oregon State University, Lawrence Berkeley National Laboratory and the Institute of Soil Landscape Research, Germany.

Findings add another twist to the ongoing debate, they add, over "the mechanisms controlling long-term stabilization of carbon in soils." Details appear in the current issue of Nature Communications.

Keiluweit explains, "Even though the presence of anaerobic microsites has long been known, this is the first study to assess their impact on Carbon stabilisation. Before, they attributed the persistence of the large carbon stocks in upland soils to other stabilisation mechanisms. Because these other stabilisation mechanisms are likely much less vulnerable to disturbance than anaerobic microsites, our research suggests that the impact of climate or land use change may release greater amounts of carbon from soils than we expected."

He and colleagues expect the new information will propel climate modelers to refine their models to better predict what may happen in the future as soils are disturbed by climate change.

Anaerobic microsites in soil are microscopic habitats lacking oxygen in which microbes are limited in their ability to metabolise soil organic matter into climate-active CO2 that is released to the atmosphere. Without oxygen, their respiration slows or comes to a stop.

For this work, Keiluweit and colleagues conducted controlled laboratory experiments in which they recreated anaerobic microsites and showed how they reduce the metabolism of organic matter and the release of CO2. They also isolated anaerobic microsites in agricultural soils in the field, where they found anaerobic microsites to have the same effect.

They write, "We show that anaerobic microsites are important regulators of soil carbon persistence, shifting microbial metabolism to less efficient anaerobic respiration and selectively protecting otherwise bioavailable, reduced organic compounds such as lipids and waxes from decomposition.

Further, shifting from anaerobic to aerobic conditions leads to a 10-fold increase in volume-specific mineralisation rate, illustrating the sensitivity of anaerobically protected carbon to disturbance. Vulnerability of anaerobically protected carbon to future climate or land use change thus constitutes a yet unrecognized soil carbon-climate feedback that should be incorporated into terrestrial ecosystem models."

Keiluweit, assistant professor in UMass Amherst's School of Earth and Sustainability, says the team's next steps include quantifying the amount of anaerobic microsites in different soil ecosystems and assessing how carbon stabilization in them is affected by variables such as the soil hydrologic regime, which is expected to change dramatically due to climate change.