Microbial Respiration in Deep Subsurface Contributes Significant Greenhouse Gas Fluxes to Atmosphere
CO2 production from soils between 2 m and 3.5 m in depth contributes ˜17% of total gas fluxes in a semiarid floodplain.
Most carbon dioxide (CO2) fluxes leaving the soil surface are commonly attributed to root and microbial respiration occurring at shallow depths (< 1 m). Less understood are respiration rates in the deeper subsurface (> 1-m depth), which contains a large inventory of organic carbon and supports an abundance of microorganisms. In this study, vertical profiles of CO2 concentrations in pore gases, measured from the soil surface down to the water table in a semiarid floodplain, were shown to have significant contributions from microbial respiration in the deeper subsurface or vadose zone, well below the rooting depth and above the water table.
The deeper vadose zone was shown to contribute a significant amount of CO2 to the total floodplain—approximately 17% of the surface CO2 flux originates from depths between 2 m and 3.5 m. These contributions are not typically accounted for in Earth system models.
CO2 fluxes from soils are often assumed to originate within shallow soil horizons (< 1-m depth), whereas relatively little is known about respiration rates at greater depths. Scientists compared measured and calculated CO2 fluxes at the Rifle floodplain along the Colorado River, and measured CO2 production rates of floodplain sediments to determine the relative importance of deeper vadose zone respiration. Calculations based on measured CO2 gradients and estimated effective diffusion coefficients yielded fluxes that are generally consistent with measurements obtained at the soil surface (326 g C m-2 yr-1). CO2 production from the 2 to 3.5-m depth interval was calculated to contribute 17% of the total floodplain respiration, with rates that were larger than some parts of the shallower vadose zone and underlying aquifer. Microbial respiration rates determined from laboratory incubation tests of the sediments support this conclusion. The deeper unsaturated zone typically maintains intermediate water and air saturations, lacks extreme temperatures and salinities, and is annually resupplied with organic carbon from snowmelt-driven recharge and by water table decline. This combination of favorable conditions supports deeper unsaturated zone microbial respiration throughout the year.
Lawrence Berkeley National Laboratory
This work was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Subsurface Biogeochemical Research program.
T. K. Tokunaga, Y. Kim, M. E. Conrad, M. Bill, C. Hobson, K. H. Williams, W. Dong, J. Wan, M. J. Robbins, P. E. Long, B. Faybishenko, J. N. Christensen, and S. S. Hubbard, “Deep vadose zone respiration contributions to carbon dioxide fluxes from a semiarid floodplain,” Vadose Zone Journal 15(7) (2016). [DOI: 10.2136/vzj2016.02.0014].