HR: 1340h
AN: B23D-1578    [Abstracts]
TI: Feedback of Ambient Air CO₂ Concentration on Soil CO₂ Efflux
AU: * Xu, L
EM: liukang.xu@licor.com
AF: Li-COR Biosciences, 4421 Superior St, Lincoln, NE 68504,
AU: McDermitt, D
EM: dayle.mcdermitt@licor.com
AF: Li-COR Biosciences, 4421 Superior St, Lincoln, NE 68504,
AU: Madsen, R
EM: rod.madsen@licor.com
AF: Li-COR Biosciences, 4421 Superior St, Lincoln, NE 68504,
AU: Demetriades-Shah, T
EM: tanvir.demetriadesshah@licor.com
AF: Li-COR Biosciences, 4421 Superior St, Lincoln, NE 68504,
AU: Garcia, R
EM: rick.garcia@licor.com
AF: Li-COR Biosciences, 4421 Superior St, Lincoln, NE 68504,
AU: Welles, J
EM: jon.welles@licor.com
AF: Li-COR Biosciences, 4421 Superior St, Lincoln, NE 68504,
AU: Furtaw, M
EM: mike.furtaw@licor.com
AF: Li-COR Biosciences, 4421 Superior St, Lincoln, NE 68504,
AB: Soil CO₂ flux (F_c) is driven largely, or in part, by the CO₂ concentration gradient across the soil surface. We show that under calm and warm night-time conditions, ecosystem respiration can lead to elevated ambient air CO₂ concentration (C_a) above the soil, which suppresses F_c, as expected from diffusion theory. We hypothesize that on warm and calm nights prolonged suppression of Fc has the effect of capping the soil, and leads to elevated soil CO₂ concentrations (C_s). When the atmosphere becomes unstable at sunrise, or when the friction velocity (U*) increases, this cap is removed by replacing air that has elevated C_a with ambient air characteristic of the well-mixed atmosphere. This can occur quite rapidly producing a large gradient between C_s and C_a, which enhances F_c, especially at sunrise. Elevated F_c can persist for one to two hours, apparently until the soil CO₂ concentration profile readjusts. We conducted a series of experiments at two field sites with different soil and vegetation types, in which we investigated the impact of ambient CO₂ concentration on F_c. Nearly continuous measurements of night-time F_c from the two sites demonstrated that F_c was negatively correlated with changes in C_a, suggesting F_c was suppressed under high C_a due to the reduced CO₂ diffusion gradient. This has the effect of increasing CO₂ storage in the soil. At sunrise, increased turbulence caused a rapid drop in C_a and an increase in F_c that preceded any increase in soil temperature, and persisted for one to two hours. We used the LI-6400 to test the hypothesis that capping the soil with elevated C_a would lead to increased F_c after C_a returned to normal levels. We allowed the chamber headspace CO₂ concentration to rise to various levels above ambient, whereupon we scrubbed the chamber air quickly back to ambient and measured F_c at ambient C_a. Measured F_c increased with increasing CO₂ concentration in the headspace prior to measurement, as predicted by a diffusion-based mechanism. Wind-induced pressure pumping was not involved. This has important implications both for chamber measurements and for ecosystem respiration. Our results suggest that respired CO₂ can accumulate in the soil profile under calm conditions. CO₂ accumulated in the soil can slowly flush out when C_a returns to the atmospheric background level as the atmosphere becomes unstable. It is likely to take much longer to flush out CO₂ accumulated in the soil profile than to exchange CO₂ accumulated in the plant canopy. This diffusion-based process might provide an explanation, in addition to U*-dependent night-time flux and pressure pumping, for the abnormally high ecosystem respiration rate at sunrise sometimes observed by the carbon flux community. Flechard, et al. (2007, Temporal changes in soil pore space CO₂ concentration and storage under permanent grassland. Agric. Forest Meterol. 142:66 ) present a similar argument, although they suggest wind-induced pressure pumping as the primary mechanism moving CO₂ out of the soil and into the atmosphere.
DE: 0428 Carbon cycling (4806)
DE: 0429 Climate dynamics (1620)
DE: 0452 Instruments and techniques
SC: Biogeosciences [B]
MN: 2007 Fall Meeting