Soil CO2 Profile Measurements Reveal an Increase in the Proportion of Old Soil Carbon over a Thirteen Year Study at a Permafrost Site in Interior Alaska
Abstract
Permafrost degradation is a primary driver of the global atmosphere-ecosystem carbon (C) cycle, with 1300-1500 Pg of stored permafrost C potentially thawing and becoming vulnerable to decomposition. The release of old permafrost C is a particular concern for climate feedback loops, because a large loss of ancient C is not expected to be countered by an increase in Arctic plant productivity. We have collected soil carbon dioxide (CO2) from two field sites near Healy, Alaska over thirteen years and determined their radiocarbon signature (Δ14C), a valuable measurement for determining the source and age of the C. A two-pool mixture model was used to distinguish sporadic pulses of old C release from modern samples. A linear model used environmental data to predict which samples were old and which were modern.
Δ14C of modern samples exhibit a yearly decrease of 5.4 ± 1.0 permil. This is a larger drop than the change of atmospheric Δ14C at the site (4.0 permil/year), so a turnover of modern carbon in the soil does not fully explain the observation. Our conclusion is that a growing proportion of soil CO2 is ancient C, originating from deep in the permafrost where it was frozen until recent degradation. We also observed pulses of old C release at some sampling dates that appear to be driven by recent precipitation events: precipitation within the last five days was an effective predictor of these pulses, while precipitation within the last fourteen days was not a significant predictor. Two important conclusions from this work are: 1) Precipitation and soil moisture dynamics play an important role in old C release, and permafrost soil C age can be highly impacted by the recent and immediate conditions at the time of sampling 2) Even outside sporadic pulses of old C release, we found evidence that permafrost degradation is accelerating the release of old C.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B23M2586E
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0702 Permafrost;
- CRYOSPHERE;
- 1615 Biogeochemical cycles;
- processes;
- and modeling;
- GLOBAL CHANGE