In describing the rationale for their investigation of this phenomenon, Dilustro et al. (2002) note that soils store approximately three times more carbon than plants, but that almost all of that carbon is transferred to the soil through plants.  They also note that plant root responses to elevated CO2 have been largely overlooked in this regard; and they thus conclude that some of the carbon that is missing from current global carbon cycle models may well be sequestered belowground.  Thus intrigued by the possibility that enhanced carbon transfer to soils via plants responding to the aerial fertilization effect of atmospheric CO2 enrichment may account for much of the carbon that exits the atmosphere each year, the scientists designed an experiment to provide some potential answers to this important question.

On a small barrier island in the northern part of the Kennedy Space Center, Florida, USA, the group of scientists erected sixteen open-top chambers around clumps of evergreen scrub oaks and associated saw palmetto shrubs that exhibit yearly nutrient cycles similar to those of many forests.  Half of the chambers were maintained at the CO2 concentration of the ambient air, while the other half - starting on 15 May 1996 - were continuously maintained at CO2 concentrations approximately 350 ppm above ambient.  In addition, in the soils of each of the sixteen chambers, the scientists inserted two minirhizotron tubes to a depth of 101 cm, through which they viewed the growth and development of the ecosystem's fine-roots at 3-month intervals, from March 1996 to December 1997, via tiny video camera systems.  The ecosystem they studied is fire-adapted and maintained with natural fire cycles of 10- to 15-year intervals, being burned most recently in February 1996, just prior to the start of the Dilustro et al. experiment.

So what was learned?  In the words of the scientists, "our hypothesis that elevated atmospheric CO2 would increase fine-root density, productivity, mortality and turnover was demonstrated."  Indeed, by the end of the 21-month study period, the fine-root length density of the resprouting trees and shrubs in the ambient-air chambers had attained a mean of 7.53 mm cm^-2 in the top 101 cm of soil, while that of the resprouting plants in the CO2-enriched chambers had attained a mean of 21.36 mm cm^-2, indicative of a CO2-induced increase of 184% in this important root property.  Concomitantly, there was also a 55% increase in ecosystem aboveground biomass; and all this happened, as the scientists describe it, "despite water and nutrient limited conditions."

What is the significance of these findings?  Dilustro et al. say "the increased rates of fine root growth coupled with no change in decomposition rate suggest a potential increased rate of carbon input into the soil."  Furthermore, their detailed fine-root data for June of 1997 indicate a mean CO2-induced increase in fine-root length density of approximately 75% in the top three-fourths of the soil profile, but an increase on the order of 125% in the bottom quarter.  Hence, there are strong indications the bottom layer of soil was being supplied with a greater proportion of extra carbon than were the upper soil layers; and just like buried treasure, buried carbon is likely to remain undisturbed for a longer period of time the deeper it is placed in the ground.

The work of Dilustro et al. thus points to the real possibility that the ongoing rise in the air's CO2 content is indeed increasing the rate at which carbon is being removed from the atmosphere and sequestered in the soils of earth's forests, and that it is thereby providing an increasing natural brake upon the rate at which the air's CO2 content would otherwise be rising, which in turn provides an increasing brake upon the potential for CO2-induced global warming.

Reference
Dilustro, J.J., Day, F.P., Drake, B.G. and Hinkle, C.R.  2002.  Abundance, production and mortality of fine roots under elevated atmospheric CO2 in an oak-scrub ecosystem.  Environmental and Experimental Botany 48: 149-159.