October 10, 2018

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Unexpected High Carbon Fluxes from the Deep Unsaturated Zone in a Semi-Arid Region

ESM land models need to incorporate deeper soil processes to improve CO2 flux predictions.

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. The researchers found and quantified 30% of carbon dioxide CO2) annual efflux to atmosphere (60% in winter) originates from below 1 m, contrary to prediction of <1% by the Earth System model (ESM) land models.

The Impact

The team contends 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 m, the researchers measured seasonal- and depth-resolved carbon inventories and fluxes, and groundwater and unsaturated zone flow rates. The researchers identified an unexpected high dissolved organic carbon (DOC) flux from the rhizosphere into the underlying unsaturated zone. Their measurements showed that ~30% of the CO2 efflux to atmosphere (60% in winter) originates from below 1 m, contrary to prediction of <1% by ESM 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 the team contends that ESM land models need to incorporate these deeper soil processes to improve CO2 flux predictions in semi-arid climate regions.

Principal Investigator

Jiamin Wan
Lawrence Berkeley National Laboratory
jwan@lbl.gov

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

The project receives support from the Subsurface Biogeochemical Research program of the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science, under contract DE-AC02- 05CH11231.

References

Wan, J., T. K. Tokunaga, W. Dong, and K. H. Williams, et al. "Deep Unsaturated Zone Contributions to Carbon Cycling in Semiarid Environments". Journal of Geophysical Research: Biosciences 123 (9), 3045-3054  (2018). https://doi.org/10.1029/2018JG004669.