Soils contain twice as much carbon as the atmosphere. Scientists believe that soils deeper than 20 centimeters account for roughly half of this stored carbon. Researchers know that warmer temperatures can stimulate microbial decomposition of this organic soil carbon. However, they are uncertain whether warming leads to a significant loss of soil carbon stocks and consequently an increase in atmospheric carbon dioxide (CO2) levels. This study found that five years of experimental warming led to a significant reduction in the carbon stock stored in deep forest soils. These results provide empirical confirmation for possible positive feedback between warming and the release of carbon from soil.
This study provided some of the first evidence that warmer temperatures lead to a significant drop in the stored carbon stock in deep forest soils. This result suggests that there can be net transfer of carbon from the soil, where it was sequestered as organic carbon, to the atmosphere, where it is released as a greenhouse gas. The experiment, in California’s Sierra Nevada forest, found that the carbon content in subsoils dropped 33 percent over five years. In addition, warming the soil led to a 30 to 35 percent increase in CO2 emissions from the soil each year. The world’s forests are currently a net sink of atmospheric CO2 and have the potential to store large amounts of atmospheric CO2 in coming decades. However, researchers need a better understanding of how deep soils will respond to warming to accurately predict and plan for long-term changes. This cycle, if confirmed over longer time scales, could constitute positive feedback to climate change.
Subsoils below 20 cm are an important reservoir in the global carbon cycle, but little is known about their vulnerability under climate change. Scientists from Lawrence Berkeley National Laboratory, Dartmouth College, the University of Zurich, and the University of California, Berkeley, conducted a field experiment by artificially heated plots of soil down to 1 meter deep by 4 degrees C. This is the amount of warming projected by century’s end in business-as-usual climate scenarios. The experimental heating mimicked natural daily and seasonal cycles. The scientists measured a statistically significant loss of subsoil carbon (−33 ± 11 percent) in warmed plots of a conifer forest after 4.5 years of whole-soil warming (4 degrees C). The loss of subsoil carbon was primarily from unprotected particulate organic matter. Warming also stimulated a sustained 30 ± 4 percent increase in soil CO2 efflux due to increased CO2 production through the whole-soil profile. These field observations of a decline in subsoil carbon stocks with warming are strong evidence for a positive soil carbon-climate feedback. This feedback could not be concluded based on increases in CO2 effluxes alone. The high sensitivity of subsoil carbon and the different responses of soil organic matter pools suggest that models must represent these heterogeneous soil dynamics to accurately predict future feedbacks to warming.
This work is supported by the Environmental System Science program (formerly the Terrestrial Ecosystem Science program) in the Department of Energy Office of Science, Office of Biological and Environmental Research, by the Swiss National Science Foundation, and by Colorado State University.