Microbial Dynamics Can Limit Soil Carbon Storage Capacity

Using data synthesis and simulations to advance a new hypothesis for why soil carbon storage appears to saturate as inputs increase

Conceptual diagram showing how soil organic carbon (SOC) can saturate under elevated carbon (C) inputs. The largest SOC pool (mineral-associated SOC) could be constrained by mineral properties or by limits to soil microbes.

[Reprinted with permission from Craig, M.E., et al. “Biological Mechanisms May Contribute to Soil Carbon Saturation Patterns.” Global Change Biology 27(12), 2633–2644 (2021). [DOI: 10.1111/gcb.15584] © 2021 John Wiley & Sons Ltd.]

The Science

Organic additions to soils increase ecosystem carbon storage, but soils have a limited capacity to store carbon. Researchers call this phenomenon “soil carbon saturation”. Normally, researchers assume that soil traits cause carbon saturation, yet microbial processes are also critical in controlling soil carbon dynamics. In this study, scientists advance a new hypothesis: soil carbon saturation can be caused by the factors that limit microbial populations. To evaluate this hypothesis, they compiled data from experiments and embedded alternative hypotheses in soil carbon models.

The Impact

Storing more carbon in the soil removes carbon dioxide (CO2) from the atmosphere. But the extent of this effect depends on how much carbon soils could hold. This study expands our understanding of the causes of soil carbon saturation and informs how we might manage soils to store more carbon. Soil carbon storage could be limited by controls on microbes, which are easier to manipulate than soil traits. Under the right conditions, soils that seem to be saturated might be able to store more carbon. This study also highlights important microbial dynamics that are missing from current models.


Increasing soil carbon storage is a key strategy to reduce atmospheric CO2. Adding organic inputs to soils increases carbon storage, but soils can only store so much carbon. This phenomenon of “soil carbon saturation” could result from properties of soil itself. For example, there is a widely assumed upper limit to soil carbon that increases with soil clay content. In this study, researchers suggest that soil carbon saturation could also be driven by constraints on soil microbes. The authors compiled data from field and laboratory experiments and found evidence of microbial population limits as organic inputs increase. Then, they simulated these limits in a soil carbon model and found that saturation could occur even without assuming an innate upper limit. The results imply that more realistic representations of microbes in soil carbon models could help us predict how soils will respond to environmental change and could help us manage soils to store more carbon.

Principal Investigator

Matthew Craig
Oak Ridge National Laboratory

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science


This research is part of the Terrestrial Ecosystem Science Scientific Focus Area at Oak Ridge National Laboratory (ORNL). It is supported by the Office of Biological and Environmental Research’s (BER) Environmental System Science (ESS) program, within the U.S. Department of Energy’s (DOE) Office of Science.


Craig, M.E., et al. "Biological Mechanisms May Contribute to Soil Carbon Saturation Patterns." Global Change Biology 27 (12), 2633–2644   (2021). https://doi.org/10.1111/gcb.15584.