Background
The authors write that "the physical protection of carbon in soil aggregates is a key mechanism affecting soil carbon dynamics (e.g. Tisdall and Oades, 1982; Six et al., 2004; Blanco-Canqui and Lal, 2004; Jastrow et al., 2007)," noting in this regard that "stabilization of soil organic carbon (SOC) in aggregates decreases its accessibility to microbes and soil fauna," thereby protecting SOC from rapid mineralization and increasing its in-soil residence time (Sollins et al., 1996; Marinissen and Hillenaar, 1997; Jastrow et al., 2007).

What was done
Sanchez-de-Leon et al., as they describe it, "conducted a 26-day laboratory incubation experiment using plant and soil materials with differential dual isotopic compositions obtained from different CO2 and ¹⁵N-labeling treatments at the Oak Ridge National Laboratory (ORNL) FACE [Free-Air CO2 Enrichment] site," where they "used crushed and sieved unlabeled soil to create four treatments: (I) soil only; (II) soil and plant material; (III) soil, plant material and the native, endogeic earthworm Diplocardia sp.; (IV) soil, plant material, and the European, epiendogeic earthworm Lumbricus rubellus," where the added plant materials consisted of both sweetgum (L. styraciflua) leaf and root litter.

What was learned
The five U.S. researchers report that (1) "overall, earthworms increased the mass of newly formed soil macro-aggregates," that (2) "most of the carbon within macro-aggregates was soil-derived," and that (3) "leaf- and root-derived carbon was found only in the treatment with L. rubellus." And so it was that they learned that "the source of carbon within macro-aggregates paralleled earthworm feeding ecologies, with endogeic earthworms feeding mostly on soil organic matter and epi-endogeic earthworms feeding on both plant residues and soil organic matter."

What it means
Sanchez-de-Leon et al. conclude from their findings that "earthworms at the ORNL-FACE site directly contribute to the formation of soil aggregates and could be an important factor contributing to the soil stabilization of increased recent carbon inputs resulting from atmospheric CO2 enrichment," one of the significant consequences of which - in the eyes of the world's climate alarmists - is to slightly reduce the rate at which the air's CO2 content continues to rise.

References
Blanco-Canqui, H. and Lal, R. 2004. Mechanisms of carbon sequestration in soil aggregates. Critical Reviews in Plant Sciences 23: 481-504.

Jastrow, J.D., Amonette, J.E. and Bailey, V.L. 2007. Mechanisms controlling soil carbon turnover and their potential application for enhancing carbon sequestration. Climatic Change 80: 5-23.

Marinissen, J.C.Y. and Hillenaar, S.I. 1997. Earthworm-induced distribution of organic matter in macro-aggregates from differently managed arable fields. Soil Biology and Biochemistry 29: 391-395.

Six, J., Bossuyt, H., Degryze, S. and Denef, K. 2004. A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics. Soil Tillage Research 79: 7-31.

Sollins, P., Homann, P. and Caldwell, B.A. 1996. Stabilization and destabilization of soil organic matter: mechanisms and controls. Geoderma 74: 65-105.

Tisdall, J.M. and Oades, J.M. 1982. Organic matter and water-stable aggregates in soils. Journal of Soil Science 33: 141-163.