The Science
Permafrost soils near the North Pole contain roughly twice the amount of carbon stored in the atmosphere today; but for now, most of this soil organic carbon is perennially frozen. However, warming in the Arctic is thawing permafrost soils and mobilizing previously frozen dissolved organic matter (DOM) from deeper soil layers to nearby surface waters. The subsequent degradation of DOM by bacteria may cause a substantial increase in CO₂ release to the atmosphere, triggering a positive feedback loop that could amplify climate change. To better understand this risk, a recent study characterized the chemical composition of DOM drained from different depths in permafrost soils and related these compositional differences to its susceptibility to biological degradation.
The Impact
The findings revealed deeper permafrost soil layers contain molecules that are more readily degraded by bacteria compared to shallow layers. However, higher degradation rates in the deep permafrost layer may not translate into a rapid increase in CO₂ release into the atmosphere. These findings could lead to more accurate global climate models and more effective strategies to mitigate climate change.
Summary
Researchers from the University of Michigan, Ann Arbor, examined permafrost layers of soils within the Imnavait Creek watershed on the North Slope of Alaska. They coupled ultra-high resolution mass spectrometry, 13C solid-state NMR, and optical spectroscopy methods with statistical analyses. Fourier transform ion cyclotron resonance mass spectrometry and 13C NMR analyses were performed at EMSL, the Environmental Molecular Sciences Laboratory, a Department of Energy national scientific user facility.
Compared to DOM from the active, annually thawed soil layer, DOM from the deeper permafrost soil layer consisted of smaller molecules with more hydrogen and less oxygen. Because these molecules are readily used by bacteria, soil carbon from the deeper permafrost layer degraded faster than soil carbon from the active layer in laboratory incubation studies. Surprisingly, the higher microbial degradation rates of DOM from the deeper permafrost layer did not yield more CO₂. This is because microbes convert soil carbon from the deeper permafrost layer to biomass faster than compared to soil carbon from the active layer. Eighty percent of deeper permafrost carbon was fixed into biomass to support cell growth and replication, compared to only twenty percent of carbon from the active layer.
The findings demonstrate microbes convert more permafrost carbon to biomass, as opposed to directly respiring it to CO₂. The implication is that permafrost carbon may not be converted to CO₂ and released into the atmosphere as quickly as previous studies indicated. Future studies will need to examine what happens to biomass once microbes die. One possibility is the biomass will be preserved for a long time in freshwater or marine sediments. This line of research could lead to improved projections of atmospheric CO₂ levels in global climate models.
Contact: Nancy Hess, nancy.hess@pnnl.gov and Nancy Washton, Nancy.Washton@pnnl.gov, EMSL; and Rose Cory rmcory@umich.edu
Funding: National Science Foundation
Publication: Ward CP and RM Cory. "Chemical composition of dissolved organic matter draining permafrost soils." Geochimica et Cosmochimica Acta. doi:10.1016/j.gca.2015.07.001
Performers/Facilities:
- University of Michigan, Ann Arbor
- EMSL