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PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Strong Regional Atmospheric 14C Signature of Respired CO2 Observed from a Tall Tower over the Midwestern United States

Brian LaFranchi

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14 September 2016

The Science
A recent study demonstrates a novel methodology for constraining the net exchange of CO2 between the landscape and atmosphere using 14CO2 observed from a tall tower in the Midwestern US. Exchanges include net ecosystem respiration (including below ground carbon), fires, and anthropogenic sources.

The Impact
The researchers determined that soil respiration of carbon drives variability in 14CO2 during the summer months and that simulations from the CASA model underestimate the biospheric 14CO2 source compared to observations at the Wisconsin Tall Tower. This approach has the potential to better constrain the long-term carbon balance of terrestrial ecosystems, the short-term impact of disturbance-based loss of carbon to the atmosphere, and highlights areas for continued land-surface/biogeochemistry model development. 

Summary
Findings showed that during the summer months the biospheric component dominates the atmospheric 14CO2 budget at the Park Falls AmeriFlux/WLEF Tall Tower in northern Wisconsin. Respiration of carbon from soils is an important component of the global carbon cycle, returning carbon previously taken up via photosynthesis over decadal time scales back to the atmosphere. For 2010, observations from 400 m above ground level indicate that the terrestrial biosphere was responsible for a 2–3 times higher contribution to total 14CO2 than predicted by the CASA terrestrial ecosystem model. This indicates that the model is underpredicting ecosystem respiration and net primary production. Based on back-trajectory analyses, this bias likely includes a substantial contribution from the North American boreal ecoregion, but transported biospheric emissions from outside the model domain cannot be ruled out. The 14CO2 enhancement also appears somewhat decreased in observations made over subsequent years, suggesting that 2010 may be anomalous. Acknowledging these caveats, the study discusses the implications of the observation/model comparison in terms of possible systematic underestimation of biospheric 14CO2 emissions, and thus turnover time of belowground carbon, in the model versus short-term anomalies in the observations.

Contacts
Daniel Stover and Jared DeForest
SC-23.1
Daniel.Stover@science.doe.gov (301-903-0289) and Jared.DeForest@science.doe.gov (301-903-1678)
Karis McFarlane
Lawrence Livermore National Laboratory
kjmcfarlane@llnl.gov (925-423-6285)
Brian LaFranchi
Now at Aclima
brian.lafranchi@gmail.com (802-310-7083)
Tom Guilderson
Lawrence Livermore National Laboratory
guilderson1@llnl.gov (925-422-1753)

Funding
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and Sandia National Laboratory, operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DEAC04-94AL85000.
This work was funded by DOE Office of Science, Office of Biological and Environmental Research, Climate and Environmental Science Division, Terrestrial Ecosystem Program (SCW1447); LLNL Lab Directed Research and Development (ERD-14-038); NOAA ESRL Global Monitoring Division; and NOAA Climate Program Office's Atmospheric Chemistry, Carbon Cycle.

Publications
LaFranchi, B.W., K.J. McFarlane, J.B. Miller, S.J. Lehman, C.L. Phillips, A.E. Andrews, P.P. Tans, H. Chen, Z. Liu, J.C. Turnbull, X. Xu, and T.P. Guilderson, “Strong Regional Atmospheric 14C Signature of Respired CO2 Observed from a Tall Tower over the Midwestern United States. Journal of Geophysical Research: Biogeosciences 122, 2275 (2016). [10.1002/2015JG003271]

Related Links
Manuscript: http://onlinelibrary.wiley.com/doi/10.1002/2015JG003271/epdf
LEF Tower Data: http://www.esrl.noaa.gov/gmd/dv/iadv/.

DOE Laboratory research; DOE-BER and LLNL LDRD; 14C analyses were performed at CAMS/LLNL.


Time series of Δ14CO2 at LEF (blue circles) from 2010 to 2012, with smoothed NWR (black solid line) and BRW (black dashed line) observations shown for reference. Residuals about the smoothed curves are on the order of the single sample measurement uncertainty (approximately 2‰). (Source: LaFranchi et al., 2016, JGR)

midsized version

(a) Time series and (b) histogram of individual observations (blue) and simulations (red) of δΔ. Model and observations are shown for 2010 only. (Source: LaFranchi et al., 2016, JGR)

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