Henning et al. (1996) grew sorghum in open-top field chambers maintained at atmospheric CO2 concentrations of approximately 355 and 710 ppm, after which some of the crop's leaf and stem tissues were applied to a Norfolk loamy sand and aerobically incubated for 70 days. This protocol revealed that the source of the plant residue, i.e., whether it was produced in ambient or CO2-enriched air, had no impact on soil carbon turnover, relative nitrogen mineralization, cumulative carbon and nitrogen mineralization or the ratio of mineralized carbon to mineralized nitrogen. Consequently, Henning et al. concluded that increasing atmospheric CO2 concentrations will likely have little effect on either the composition or decomposition of field residues of sorghum. Nevertheless, they also opined that "since CO2 enrichment results in increased photosynthetic carbon fixation, the possibility exists for increased soil carbon storage."

In a subsequent test of this hypothesis, Torbert et al. (1998) added residues of sorghum grown at ambient and twice-ambient atmospheric CO2 concentrations to soil plots exposed to normal air to see if the decomposition rates of the two types of residue differed. Some of the plots received equal amounts of the two types of residue, while others received larger amounts of the residue that was produced in the CO2-enriched air that were reflective of the greater amounts of biomass produced in that treatment. Then, measurements of carbon evolution from the soil plots were made at 3, 14, 30 and 60 days after the residue additions to indirectly determine how much carbon remained in the soil.

In the case where equal amounts of the two types of residue were supplied to the soil plots, and at all measurement times with the exception of day 3, significantly lower amounts of carbon were lost from the soils that were amended with the crop residues that were produced in the elevated CO2 treatment. Even more impressive was the fact that when the soils that were amended with residues produced in the elevated CO2 treatment received an extra supply of such residues that was equal to the extra amount of biomass produced in that treatment, they always exhibited significantly less carbon evolution than that exhibited by the soils amended with the residues produced in ambient air. In fact, after 60 days the cumulative amount of carbon evolved from the greater amount of high-CO2-produced residues was about 40% less than that evolved from the smaller amount of ambient-air-produced residues, implying that considerably more carbon may well be sequestered in soils planted to sorghum in a CO2-enriched world of the future than is the case today.

In perhaps the longest such study of its type ever to be conducted on sorghum, Prior et al. (2004) grew the agriculturally-important C4 plant from seed to maturity for five consecutive growing seasons within open-top chambers maintained at atmospheric CO2 concentrations of either 360 or 720 ppm. The soil in which the plants grew had been fallow for more than 25 years prior to the start of the study and was located within a huge outdoor bin. At the end of each growing season, aboveground non-yield residues (stalks, chaff and 10% of the grain yield) were allowed to remain on the surfaces of the plots to simulate what occurs in no-tillage farming. Measurements of certain soil properties made at the end of the experiment were then compared with similar measurements conducted at the beginning of the study.

Among other things, the researchers found that the elevated CO2 treatment increased soil aggregate stability, which is known to promote belowground carbon sequestration, and that it increased total soil carbon content by fully 16%. These results were described by Prior et al. as "CO2-induced benefits," which they truly are, in and of their own right, but also because they have a tendency to slow the rate of rise of the air's CO2 concentration and somewhat mitigate the potential for global warming.

References
Henning, F.P., Wood, C.W., Rogers, H.H., Runion, G.B. and Prior, S.A. 1996. Composition and decomposition of soybean and sorghum tissues grown under elevated atmospheric carbon dioxide. Journal of Environmental Quality 25: 822-827.

Prior, S.A., Runion, G.B., Torbert, H.A. and Rogers, H.H. 2004. Elevated atmospheric CO2 in agroecosystems: Soil physical properties. Soil Science 169: 434-439.

Torbert, H.A., Prior, S.A., Rogers, H.H. and Runion, G.B. 1998. Crop residue decomposition as affected by growth under elevated atmospheric CO2. Soil Science 163: 412-419.