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Atmospheric CO2 Enrichment Effects on Soil Physical Properties in No-Till Agriculture
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.

The authors note that "enhanced aboveground crop growth under elevated CO2, leading to more soil surface residue and greater percent ground cover (Prior et al., 1997) coupled with positive shifts in crop root systems (Prior et al., 2003), may have the potential to alter soil structural characteristics."  Hence, they decided to see if this inference was indeed true, and if it was true, to see if elevated atmospheric CO2 concentrations tended to enhance or degrade soil physical properties.

What was done
Prior et al. grew plots of soybean (Glycine max (L.) Merr. cv. Stonewall) and sorghum (Sorghum bicolor (L.) Moench cv. Savanna 5) plants 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, soybean pod hulls and sorghum chaff), including 10% (by weight) of the grain yield, were allowed to remain on the surfaces of the plots to simulate no-tillage farming.  Measurements of certain soil properties made at the beginning of the experiment were then compared with similar measurements conducted at its conclusion.

What was learned
The elevated CO2 (1) had no effect on soil bulk density in the sorghum plot, but lowered it in the soybean plot by approximately 5%, (2) had no effect on soil saturated hydraulic conductivity in the sorghum plot, but increased it in the soybean plot by about 42%, (3) increased soil aggregate stability in both plots, but by a greater amount in the soybean plot, and (4) increased total soil carbon content by 16% in the sorghum plot and 29% in the soybean plot.  Consequently, the soils of both plots experienced some improvements in response to the experimental doubling of the air's CO2 content, although there were more and greater improvements in the soybean plot than in the sorghum plot.

What it means
Prior et al. say their results "indicate potential for improvements in soil carbon storage, water infiltration and soil water retention, and reduced erosion," which valuable positive consequences they rightly describe as "CO2-induced benefits."

Prior, S.A., Rogers, H.H., Runion, G.B., Torbert, H.A. and Reicosky, D.C.  1997.  Carbon dioxide-enriched agro-ecosystems: Influence of tillage on short-term soil carbon dioxide efflux.  Journal of Environmental Quality 26: 244-252.

Prior, S.A., Torbert, H.A., Runion, G.B. and Rogers, H.H.  2003.  Implications of elevated CO2-induced changes in agroecosystem productivity.  Journal of Crop Production 8: 217-244.

Reviewed 1 December 2004