05/13/2019

Millennial and Fast-Cycling Arctic Soil Carbon are Equally Sensitive to Warming

Radiocarbon-based evidence from a soil warming experiment was used to understand carbon decomposition.

The Science

This study investigated the effects of warming on Arctic soil carbon and showed that decomposition rates of fast-cycling and slow-cycling soil carbon are equally temperature sensitive. The study used an incubation experiment and a novel method for analyzing radiocarbon content to evaluate soil carbon age and decomposability and to disentangle the effects of warming and substrate depletion on carbon mineralization.

The Impact

In soils from Utqiagvik (formerly Barrow), Alaska, ancient soil carbon was highly vulnerable to warming, with no relationship between temperature sensitivity and historical cycling rate. When soils were thawed and oxygen was not limiting, carbon that had been stored for centuries or millennia was poorly protected against microbial decomposition.

Summary

Intact (nonhomogenized) soil samples from Utqiagvik, Alaska, were sequentially incubated at 5°C and 10°C at Lawrence Berkeley National Laboratory. To account for substrate depletion as the experiment progressed, a third incubation was performed at 5°C. Carbon dioxide (CO₂) production rates and natural abundance Δ¹⁴C of CO₂ were measured after each incubation to evaluate vulnerability to warming of slow-cycling and fast-cycling soil carbon pools. Based on Δ¹⁴C values from the first incubation, very old soil carbon was readily decomposable when soils were thawed and aerobic. A novel regression technique was used to estimate temperature sensitivities using bulk (measured) CO₂ production rates, and rates partitioned with radiocarbon into fast-cycling (carbon age = 50 years) and slow-cycling (carbon age = 5,000 years) pools. No difference in temperature sensitivity was found between fast-cycling and slow-cycling carbon. These findings suggest that mechanisms other than chemical recalcitrance mediate the effect of warming on soil carbon mineralization.

Principal Investigator(s)

Margaret Torn
Lawrence Berkeley National Laboratory
[email protected]

Funding

This research was conducted through the Next-Generation Ecosystem Experiments (NGEE)–Arctic project, which is supported by the Office of Biological and Environmental Research within the U.S. Department of Energy’s Office of Science.

References

Vaughn, L. J. S., and M. S. Torn “¹⁴C evidence that millennial and fast-cycling soil carbon are equally sensitive to warming.” Nature Climate Change 9, 437–38 (2019). [DOI: 10.1038/s41558-019-0468-y]