Background
"Soil aggregates," in the words of the authors, "are groups of primary particles that adhere to each other more strongly than to surrounding soil particles (Martin et al., 1955)."  With respect to their many functions, they report that "soil structure and water-stable aggregation are crucial for facilitating water infiltration, soil-borne aspects of biogeochemical cycling processes, success of sustainable agriculture, and for providing resistance against erosional loss of soil (Oades, 1984; Elliott and Coleman, 1988; Van Veen and Kuikman, 1990; Bethlenfalvay and Lindermann, 1992; Daily, 1995; Arshad et al., 1996; Coleman, 1996; Jastrow and Miller, 1997; Young et al., 1998)."

What was done
In a FACE study (daylight ambient CO2 = 373 ppm, daylight elevated CO2 = 566 ppm) conducted in a field planted to sorghum (Sorghum bicolor L. cv. Dekalb DK54), where plants were grown in adequately fertilized soil and there were well-watered (wet) and water-stressed (dry) irrigation treatments, the authors measured the effects of elevated CO2 on the hyphal growth of arbuscular mycorrhizal fungi (AMF), two fractions of glomalin - "a soil protein produced by AMF that is very tightly correlated with soil aggregate water stability (Wright and Upadhyaya, 1998; Wright et al., 1999)" - and the production of water-stable soil aggregates.

What was learned
Fungal hyphae lengths were dramatically increased by the 50% increase in the air's CO2 concentration: by about 120% in the wet irrigation treatment and by approximately 240% in the dry treatment.  The mass of water-stable soil aggregates was also increased by the biological effects of the extra CO2 in the air: by 40% in the wet treatment and 20% in the dry treatment.  In addition, the researchers report that "two fractions of glomalin and AMF hyphal lengths were all positively correlated with soil aggregate water stability."

What it means
In considering the results of their study, Rillig et al. conclude it was "thus demonstrated for the first time that elevated CO2 can affect soil aggregation in an agricultural system," where "a soil stabilizing effect of CO2 would be clearly advantageous."  In this regard, they note that "during the last 40 years, nearly one third of the world's arable land was lost by erosion, with a current loss rate of more than 10 million hectares per year," citing Pimental et al. (1995).  Hence, they note that "a CO2-mediated increase of soil aggregate stability could be of particular importance in agroecosystems."

References
Arshad, M.A., Lowery, B. and Grossman, B.  1996.  Physical tests for monitoring soil quality.  In: Methods for Assessing Soil Quality, SSSA Special Publication 49.  Soil Science Society of America, Madison, Wisconsin, USA, pp. 123-141.

Bethlenfalvay, G.J. and Linderman, R.G.  1992.  Mycorrhizae in Sustainable Agriculture.  ASA Special Publication 54. American Society of Agronomy, Madison, Wisconsin, USA.

Coleman, D.C.  1996.  Fundamentals of Soil Ecology.  Academic Press, San Diego, California, USA.

Daily, G.C.  1995.  Restoring value to the world's degraded lands.  Science 269: 350-354.

Elliott, E.T. and Coleman, D.C.  1988.  Let the soil work for us.  Ecological Bulletin 39: 23-32.

Jastrow, J.D. and Miller, R.M.  1997.  Soil aggregate stabilization and carbon sequestration: feedbacks through organomineral associations.  In: Lal, R. et al., Eds.  Soil Processes and the Carbon Cycle.  CRC Press, Boca Raton, Florida, USA, pp. 207-223.

Martin J.P., Martin, W.P., Page, J.B., Ranley, W.A. and De Ment, J.D.  1955.  Soil aggregation.  Advances in Agronomy 7: 1-37.

Oades, J.M.  1984.  Soil organic matter and structural stability: mechanisms and implications for management.  Plant and Soil 76: 319-337.

Pimentel, D., Harvey, C., Resosudarmo, P. et al.  1995.  Environmental and economic costs of soil erosion and conservation benefits.  Science 267: 1117-1123.

Van Veen, J.A. and Kuikman, P.J.  1990.  Soil structural aspects of decomposition of organic matter by micro-organisms.  Biogeochemistry 11: 213-233.

Wright, S.F., Starr, J.L. and Paltineau, I.C.  1999.  Changes in aggregate stability and concentration of glomalin during tillage management transition.  Soil Science Society of America Journal 63: 1825-1829.

Wright, S.F. and Upadhyaya, A.  1998.  A survey of soils for aggregate stability and glomalin, a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi.  Plant and Soil 198: 97-107.

Young, I.M., Blanchart, E., Chenu C. et al.  1998.  The interaction of soil biota and soil structure under global change.  Global Change Biology 4: 703-712.