Soils Could Release Much More Carbon than Expected as Climate Warms

The Science

Newswise — Soils contain more carbon than the atmosphere, so how soils respond to warming is crucial to knowing how carbon fluxes in a changing climate. For the first time, scientists explored what happens when the heating extends to a depth of 100 centimeters (~3 feet). Warming the entire depth by several degrees (4 oC) increased annual soil respiration by 34 percent to 37 percent—considerably more than observed before. More than 40 percent of this increase came from below the depth considered by most studies.

The Impact

The impact of warming on soil carbon dioxide flux is a major uncertainty in climate feedbacks. The whole-soil warming experiment found a larger respiration response than (1) many other controlled experiments, which may have missed the response of deeper soils; and (2) most models. Thus, the strength of the soil carbon–climate feedback may be underestimated.

Summary

Soil organic carbon harbors three times as much carbon as Earth’s atmosphere, more than half of that below the 20-centimeter (0.66-foot) depth. The response of whole-soil profiles to warming has not been tested in situ. In this deep warming experiment in mineral soil, carbon dioxide production from all soil depths increased significantly with 4 °C warming; annual soil respiration increased by 34 percent to 37 percent. All depths responded to warming with similar temperature sensitivities, driven by decomposition of decadal-aged carbon. Whole-soil warming reveals a larger soil respiration response than many in situ experiments, most of which only warm the surface soil, and models. In this year-round experiment, plots were warmed by a ring of 22 vertical heating cables installed to 2.4-meter depth (~7.9 foot). Three plots (3-meter (~10-foot) diameter each) were warmed, and three served as controls. Soil respiration was measured by chambers at the surface and gas tubes at five depths. The radiocarbon content of carbon dioxide and soil fractions suggests that respiration—and its warming response—was dominated by decadal cycling carbon.

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Science program.

Publications

C. E. Hicks Pries, C. Castanha, R. C. Porras, and M. S. Torn, “The whole-soil carbon flux in response to warming.” Science355(6332), 1420-1423 (2017). [DOI: 10.1126/science.aal1319]