What was done
Working with undisturbed soil cores with and without visible wheat residues that were extracted at the conclusion of the third year of a mini-FACE (free-air CO2 enrichment) experiment conducted in a field planted annually to spring wheat (Triticum aestivum L.) near Hohenheim, Germany, the authors studied the effect of elevated atmospheric CO2 concentration (an extra 160 ppm) on the decomposition of the wheat residues present in the soil by measuring CO2 evolution from the cores, as well as the leaching of inorganic and organic carbon from them, during 191 days of core incubations in the laboratory.

What was learned
Marhan et al. report that cumulative residue decomposition was not affected by elevated CO2 when no wheat residues were visible in the cores. When such residues were visible, however, decomposition was found to be "significantly lower" (by 19%) in the elevated compared to the ambient CO2 treatment, which for the more common 300-ppm degree of atmospheric CO2 enrichment roughly translates to a decomposition reduction of 36%. In addition, they found that more dissolved inorganic carbon (DIC) was leached from the elevated CO2 treatment cores, both with and without visible plant residues, than from similar cores from the ambient CO2 treatment (47.2% and 29.5%, respectively, for their degree of CO2 enrichment, which equates to about 88% and 55%, respectively, for a 300-ppm increase in atmospheric CO2 concentration); and they say that these extra amounts of DIC represent "an additional possible mechanism for carbon sequestration in soils of arable cropping systems under future elevated CO2 concentrations." Finally, on top of everything else, they report that stubble and root biomass tended to be higher by 12.0 and 9.44%, respectively, in soil cores taken from the elevated CO2 plots at the end of the study, which equates to approximate stubble and root biomass enhancements of 22% and 18%, respectively, for a 300-ppm increase in atmospheric CO2 concentration.

What it means
With respect to the potential for enhanced carbon sequestration in wheat (and other cereal-crop) fields in a CO2-enriched world of the future, the six scientists conclude that "increased input of plant residues and reduced decomposition of plant-derived carbon" are, indeed, "possible mechanisms for enhanced carbon sequestration under elevated atmospheric CO2 concentration."