U.S. Department of Energy Office of Biological and Environmental Research

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Unexpected High Carbon Fluxes from the Deep Unsaturated Zone in a Semi-Arid Region
Published: September 04, 2018
Posted: March 22, 2019

The Science
Understanding of terrestrial carbon cycling has relied primarily on studies of top soils that are typically characterized to depths shallower than 0.5 m. We found and quantified 30% of CO2 annual efflux to atmosphere (60% in winter) originates from below 1 m, contrary to prediction of <1% by the ESM land models.

The Impact
We contend that ESM land models need to incorporate these deeper soil processes to improve CO2 flux predictions in semi-arid climate regions.

Summary
Understanding of terrestrial carbon cycling has relied primarily on studies of topsoils that are typically characterized to depths shallower than 0.5 m. At a semi-arid site, instrumented down to 7 meters, we measured seasonal- and depth-resolved carbon inventories and fluxes, and groundwater and unsaturated zone flow rates. We identified an unexpected high dissolved organic carbon (DOC) flux from the rhizosphere into the underlying unsaturated zone. We measured that ~30% of the CO2 efflux to atmosphere (60% in winter) originates from below 1 m, contrary to prediction of <1% by Earth System Model land models. The seasonal DOC influx and favorable temperatures, moisture and oxygen availability in deeper unsaturated zone sustained the respirations of deeper microbial communities and roots. These conditions are common characteristics of many subsurface environments; thus we contend that ESM land models need to incorporate these deeper soil processes to improve CO2 flux predictions in semi-arid climate regions.

Contacts (BER PM)
David Lesmes, SC-23.1
david.lesmes@science.doe.gov

(PI Contact)
Jiamin Wan, Lawrence Berkeley National Laboratory
jwan@lbl.gov

Funding
U.S. Department of Energy (DOE) Subsurface Biogeochemical Research Program, DOE Office of Science, Office of Biological and Environmental Research, under contract DE-AC02- 05CH11231.

Publications
Wan, J., Tokunaga, T.K., Dong, W., Williams, K.H., Kim, Y., Conrad, M.E., Bill, M., Riley, W.J., Susan S.H., “Deep unsaturated zone contributions to carbon cycling in semiarid environments.” Journal of Geophysical Research: Biogeosciences 123(9), 3045-3054 (2018). [DOI:10.1029/2018JG004669]

Topic Areas:

  • Research Area: Earth and Environmental Systems Modeling
  • Research Area: Subsurface Biogeochemical Research
  • Research Area: Carbon Cycle, Nutrient Cycling

Division: SC-23.1 Climate and Environmental Sciences Division, BER

 

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