Nocturnal soil CO2 uptake and its relationship to sub-surface soil and ecosystem carbon fluxes in a Chihuahuan Desert shrubland

Authors: Hamerlynck, Erik; Scott, Russell - Russ; SÁNCHEZ-CAÑETE, E. - University De Granada; BARRON-GAFFORD, G. - University Of Arizona

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2013
Publication Date: 12/6/2013
Citation: Hamerlynck, E.P., Scott, R.L., Sánchez-Cañete, E.P., Barron-Gafford, G.A. 2013. Nocturnal soil CO2 uptake and its relationship to sub-surface soil and ecosystem carbon fluxes in a Chihuahuan Desert shrubland. Journal of Geophysical Research-Biogeosciences. 118:1593–1603. DOI :10.1002/2013JG002495.

Interpretive Summary: All desert systems experience predictable, long-lasting dry spells, yet, despite their prevalence, little attention has been given to ecosystem carbon dioxide fluxes in deserts over annual drought periods. We continuously measured soil surface respiration under and between shrubs, soil carbon dioxide concentrations and fluxes across a range of soil depths, and ecosystem carbon dioxide flux in a Chihuahuan Desert shrubland over a three month dry period before the 2012 summer monsoon season. We found that under these dry conditions, soil respiration rates were often negative over the night, indicating soil carbon uptake. Nocturnal soil carbon uptake depended on how strong soil to air temperature differences were, and these conditions were more common in exposed intercanopy locations than under shrubs. While soils were taking up carbon dioxide, our soil CO2 profiles showed that CO2 at 2cm was actually below atmospheric levels, which was likely due to soil carbonate dissolution. In the morning, soil respiration rates became positive, but did so to a greater degree and faster than fluxes at different soil depths. This “out-gassing” is probably due to CO2 at and just below the soil surface being released as carbonate precipitates out. This process is probably aided by atmospheric turbulence that occurs as the air and soil surface heat up in the morning, and resulted in much higher soil respiration rates than would be predicted from prevailing temperatures. These results show inorganic carbon dynamics are the dominant feature in ecosystem gas exchange over annually re-occurring drought, and these will likely respond strongly to continued atmospheric carbon dioxide increases, as well as the warmer and drier pre-monsoon periods predicted across the Southwest U.S.

Technical Abstract: Despite their prevalence, little attention has been given to quantifying aridland soil and ecosystem carbon fluxes over prolonged, annually occurring dry periods. We measured surface soil respiration (Rsoil), volumetric soil moisture and temperature in inter- and under-canopy soils, sub-surface soil CO2 profiles and fluxes (Fs), and net ecosystem CO2 exchange (NEE) over a three-month hot and dry period in a Chihuahuan Desert shrubland. Nocturnal Rsoil was frequently negative (from the atmosphere into the soil), a form of inorganic carbon exchange infrequently observed in other deserts. Negative Rsoil depended on air-soil temperature gradients that were more frequent and stronger in intercanopy soils. Daily integrated Rsoil was always positive in both locations, but was lower in inter-canopy soils due to nocturnal uptake and more limited positive response to small, scattered rains. Negative nighttime NEE often co-occurred with negative Rsoil. Sub-surface [CO2] profiles associated with negative Rsoil indicated sustained carbonate dissolution lowered shallow-soil [CO2] below atmospheric levels. In the morning, positive surface Rsoil started earlier, and increased faster than shallow-soil Fs, which was bi-directional, with upward flux toward the surface and downward flux into deeper soils. These dynamics are consistent with carbonate precipitation in conjunction with convection-assisted CO2 out-gassing from warming air and soil temperatures, and produced a pronounced diurnal Rsoil temperature hysteresis. We concluded that abiotic nocturnal soil CO2 uptake, though a small carbon sink, modulates dry-season ecosystem-level carbon dynamics. Moreover, these abiotic carbon dynamics may be affected by higher atmospheric carbon dioxide levels and predictions of hotter, drier pre-monsoon drought periods.