Reference
Allard, V., Newton, P.C.D., Lieffering, M., Soussana, J.-F., Carran, R.A. and Matthew, C. 2005. Increased quantity and quality of coarse soil organic matter fraction at elevated CO2 in a grazed grassland are a consequence of enhanced root growth rate and turnover. Plant and Soil 276: 49-60.
What was done
Working with a temperate grassland on the North Island of New Zealand that had been under permanent grazing by sheep, cattle and goats since at least 1940, the authors measured above- and below-ground plant growth and litter production, along with root turnover and soil particulate organic matter quantity and quality, after almost four years of exposure to an extra ~105 ppm of atmospheric CO2 (a target concentration of 475 ppm) in a moderate-term FACE experiment.
What was learned
Allard et al. report that aboveground herbage biomass and leaf litter production were not altered by elevated CO2, but that root growth rate and turnover "were strongly stimulated by CO2 particularly at low soil moisture contents during summer." As a result of the root responses, they also found that "significantly more plant material was returned to the soil under elevated CO2 leading to an accumulation of coarse (>1 mm) particulate organic matter (POM)," together with a similar but not-yet-significant trend in fine POM. In addition, they found there was a CO2-induced lowering of POM carbon/nitrogen ratio, which they "attributed to the higher proportion of legumes in the pasture under elevated CO2."
What it means
The six New Zealand and French researchers say their results "show that in grazed pastures with high plant species diversity we might expect extra carbon sequestration in soil organic matter mainly through an increase in carbon input rather than a decreasing quality of accumulating organic matter." That they could detect the changes they did over so short a time interval, and with so small an increase in the atmosphere's CO2 concentration, is truly amazing; but it is just one more example of the similar findings of Jastrow et al. (2005) , plus those of the many studies the latter scientists reviewed in their meta-analysis of the subject. In addition, Allard et al. found indications of increased soil nitrogen in their CO2-enriched treatment, just as Jastrow et al. did, reinforcing the latter group's conclusions about this subject too.
Reference
Jastrow, J.D., Miller, R.M., Matamala, R., Norby, R.J., Boutton, T.W., Rice, C.W. and Owensby, C.E. 2005. Elevated atmospheric carbon dioxide increases soil carbon. Global Change Biology 11: 2057-2064.