Soils Could Release Much More Carbon Than Expected as Climate Warms

The Science

Scientists created a field experiment in a conifer forest in California to explore, for the first time, what happens to organic carbon trapped in soil when all soils are warmed. In this case, the soil layers extended to a depth of 100 cm. Warming the whole profile by 4°C increased annual soil respiration by 34% to 37%. More than 40% of this increase in respiration came from below a 15-cm depth (i.e., below the depth considered by most studies).

The Impact

The impact of warming on soil carbon dioxide (CO₂) flux is a major uncertainty in climate feedbacks. This 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 20-cm depth. The response of whole-soil profiles to warming has not been tested in situ. In this deep warming experiment in mineral soil, CO₂ production from all soil depths increased significantly with 4°C warming; annual soil respiration increased by 34% to 37%. 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-m depth. Three plots (3-m diameter each) were warmed, and three served as controls. Soil respiration was measured by chambers at the surface and gas tubes at five depths. Radiocarbon content of CO₂ and soil fractions suggests that respiration—and its warming response—was dominated by decadal cycling carbon.

Principal Investigator(s)

Margaret S. Torn
Lawrence Berkeley National Laboratory
[email protected]

Caitlin Hicks Pries
Lawrence Berkeley National Laboratory
[email protected]

Funding

This material is based on work supported by the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science, under contract number DE-AC02-05CH11231.

References

Hicks Pries, C.E., C. Castanha, R.C. Porras, and M.S. Torn. “The whole-soil carbon flux in response to warming.” Science (Early Online Research March 9, 2017) 355(6332), 1420–1423 (2017). DOI:10.1126/science.aal1319].