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Six Years of Soil Carbon Dynamics in the Duke Forest FACE Study
Reference
Lichter, J., Barron, S.H., Bevacqua, C.E., Finzi, A.C., Irving, K.F., Stemmler, E.A. and Schlesinger, W.H.  2005.  Soil carbon sequestration and turnover in a pine forest after six years of atmospheric CO2 enrichment.  Ecology 86: 1835-1847.

What was done
The authors review what has been learned about the effects of an atmospheric CO2 enrichment of 200 ppm on the soil carbon (C) dynamics of Duke Forest (an aggrading loblolly pine stand near Chapel Hill, North Carolina, USA) over the first six years of the long-term FACE experiment being conducted there.

What was learned
Over the first six years of the study, organic C accumulated in the forest floor of the elevated CO2 plots at a rate that was 52 16 g C m-2 yr-1 greater than would have been expected during reforestation under ambient CO2 conditions, as represented by the rate of C accumulation in the forest floor of the ambient CO2 plots.  This additional C sink, in the words of the authors, "resulted from increased C inputs of 50 30 g C m-2 yr-1 to the forest floor in response to CO2 enhancement of primary production."  Since there was "no evidence that the overall rate of decomposition of the forest floor decreased under the elevated CO2 treatment," they concluded that "the additional C sink in the forest floor of the elevated CO2 treatment ... is wholly dependent on the net primary production enhancement and increased C inputs," which after a total of six years have increased the forest floor's organic C content by approximately 27%, as best we can determine from the plotted data.  What is more, the data give no indication that this trend may be on the verge of declining anytime soon.

With respect to the underlying mineral soil, Lichter et al. say they could detect no statistically significant treatment effects on the C content of the bulk mineral soil or the intra-aggregate particulate organic matter and mineral-associated organic matter fractions after six years of CO2 enrichment.  Nevertheless, there was a nearly statistically significant (P = 0.11) increase of 18.5% in the free light fraction of the organic matter in the top 15 cm of the soil profile, as well as a 3.9% increase in the total intra-aggregate particulate organic matter there; and the sum of the organic C in these two categories plus the mineral-associated organic C was 11.5% greater in the CO2-enriched plots than in the ambient treatment plots.

What it means
Although the authors are somewhat pessimistic and continue to believe that "forest soils are unlikely to sequester significant additional quantities of atmospheric C associated with CO2 fertilization because of the low rates of C input to refractory and protected soil organic matter pools," the CO2-enriched trees of their study continue to demonstrate a large and unabated growth advantage over the ambient-CO2 trees, and both the forest floor and the surface soil horizon beneath the CO2-enriched trees continue to accumulate more organic C than the forest floor and surface soil horizon beneath the ambient-CO2 trees.  Perhaps as time passes, the still-unstimulated refractory and protected soil organic matter pools of which Lichter et al. write will begin to show similar responses as well.  Indeed, the likelihood of such occurring is presaged by fact that the forest-floor C/N ratio in the CO2-enriched treatment has been declining relative to that of the ambient treatment ever since the start of the experiment; and this divergence may reflect greater relative proportions of woody debris and refractory carbon compounds entering the soils of the CO2-enriched plots relative to the soils of the ambient-CO2 plots as both treatments mature.  This interpretation of the data, if correct, would ultimately be expected to vindicate our more optimistic view of the situation, which provides a strong rationale for continuing the experiment for as long as is humanly and economically possible.

Reviewed 28 September 2005