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Impact of Elevated Atmospheric CO2 on Soil Carbon Beneath a Sorghum Crop
Cheng, L., Leavitt, S.W., Kimball, B.A., Pinter Jr., P.J., Ottman, M.J., Matthias, A., Wall, G.W., Brooks, T., Williams, D.G. and Thompson, T.L. 2007. Dynamics of labile and recalcitrant soil carbon pools in a sorghum free-air CO2 enrichment (FACE) agroecosystem. Soil Biology & Biochemistry 39: 2250-2263.

What was done
In a two-year free-air CO2 enrichment (FACE) study of sorghum (Sorghum bicolor (L.) Moench), which was conducted near Phoenix, Arizona (USA), the authors studied the dynamics of soil organic carbon (SOC) pools comprised of labile and recalcitrant SOC of short and long mean residence time (MRT), respectively, under Control conditions (360 ppm CO2) and FACE conditions (560 ppm CO2) together with water-adequate (wet) and water-deficient (dry) treatments.

What was learned
Because soils typically contain large amounts of carbon compared to what they sequester annually, it is difficult to measure changes in total SOC content over periods of only a few years; and the study of Cheng et al. proved no exception to this general rule, as no significant differences in total SOC could be detected between the Control and FACE treatments over the two years of the sorghum experiment.

Nevertheless, much was learned by other means, such as stable-carbon isotopic (δ13C) tracing, which revealed that 53% of the final SOC in the FACE plot was in the recalcitrant or long MRT carbon pool and 47% in the labile or short MRT pool, whereas in the Control plot 46% and 54% of the final SOC was in the recalcitrant and labile pools, respectively, indicating, in the words of the authors, that "elevated CO2 transferred more SOC into the slow-decay carbon pool." Also, as the researchers continue, "isotopic mixing models revealed that increased new sorghum residue input to the recalcitrant pool mainly accounts for this change, especially for the upper soil horizon (0-30 cm) where new carbon in recalcitrant soil pools of FACE wet and dry treatments was 1.7 and 2.8 times as large as that in respective Control recalcitrant pools." Likewise, Cheng et al. state that "old C in the recalcitrant pool under elevated CO2 was higher than that under ambient CO2, indicating that elevated CO2 reduces the decay of the old C in [the] recalcitrant pool."

What it means
Because "higher recalcitrant C content and lower labile C content in the soils were detected under elevated CO2 relative to ambient CO2 treatments, suggesting that SOC under elevated CO2 becomes more stable against chemical and biological degradation," the ten US scientists say their results "imply that terrestrial agroecosystems may play a critical role in sequestering atmospheric CO2 and mitigating harmful CO2 under future atmospheric conditions."

Reviewed 19 September 2007