A Slippery Slope: Soil Carbon Destabilization
Carbon gain or loss depends on the balance between competing biological, chemical, and physical reactions.
Despite a breadth of research on carbon accrual and persistence in soils, scientists lack a strong, general understanding of the mechanisms through which soil organic carbon (SOC) is destabilized in soils. In a new review article, researchers synthesized principles of soil chemistry, physics, and biology to explain carbon loss in soils. They found that destabilization does not equal stabilization in reverse. Rather, carbon gain or loss depends on the balance among competing biological, chemical, and physical reactions that can be altered by changes in weather and temperature.
Rates of soil carbon respiration are increasing with current changes in climate and land use. Therefore, understanding destabilization processes in the soil carbon cycle is imperative. This review informs a more robust understanding of the processes that result in carbon loss and feedbacks to the Earth system. With this context, empirical and computational scientists can target better questions about the potential for soils to affect climate through the carbon cycle, which is important for improving predictive biogeochemical and climate models.
Most empirical and modeling research on soil carbon dynamics focuses on processes that control and promote carbon stabilization. However, the mechanisms through which SOC is destabilized in soils may be even more important to understand. Destabilization processes occur as SOC shifts from a “protected” or passive state, to an “available” or active state. In the available state, microbes can transform soil carbon to gaseous or soluble forms that are then lost from the soil.
The reviewers, from Pacific Northwest National Laboratory, Dartmouth College, and Oregon State University, considered two well-known phenomena—soil carbon priming and the Birch effect—to show how different mechanisms interact to increase carbon losses. They categorized carbon destabilization processes into three general categories: (1) release from physical occlusion through processes such as tillage, bioturbation, or freeze-thaw and wetting-drying cycles; (2) carbon desorption from soil solids and colloids; and (3) increased carbon metabolism by microbes.
By considering the different physical, chemical, and biological controls as processes that contribute to SOC destabilization, researchers can develop new hypotheses about the persistence and vulnerability of carbon in soils and make more accurate and robust predictions of soil carbon cycling in a changing environment.
Pacific Northwest National Laboratory
V. L. Bailey was supported by the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science. Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC05-557 76RL01830. K. Lajtha was supported by National Science Foundation DEB-1257032.
Bailey, V., C. Hicks Pries, and K. Lajtha. “What do we know about soil carbon?” Environmental Research Letters 14(8), 083004 (2019). DOI: 10.1088/1748-9326/ab2c11