What was done
The authors carried out a micro-morphological study of structural development and organic matter distribution in two calcareous marine loam soils on which pear trees had been grown for 45 years.  The soil of one of the Dutch orchards exhibited little to no earthworm activity, while the soil of the other orchard exhibited high earthworm activity.  This difference was the result of different levels of heavy metal contamination of the soils of the two orchards due to the prior use of different amounts of fungicides.

What was learned
The absence of earthworms led to topsoil compaction, restricted liter incorporation into the mineral portion of the soil, less fragmentation of particulate organic matter, and restricted mixing of organic matter with the mineral soil's clay fraction.  Furthermore, without earthworms there are no earthworm casts; and the authors point out that "the rate of organic matter decomposition can be decreased in worm casts compared to bulk soil aggregates (Martin, 1991; Haynes and Fraser, 1998)."

What it means
Based on their own results, as well as those of others, Jongmans et al. conclude that "earthworms play an important role in the intimate mixing of organic residues and fine mineral soil particles and the formation of organic matter-rich micro-aggregates and can, therefore, contribute to physical protection of organic matter, thereby slowing down organic matter turnover and increasing the soil's potential for carbon sequestration."/p>

These points take on added significance when it is realized that elevated levels of atmospheric CO2 tend to increase earthworm populations and activities, as has been demonstrated by Zaller and Arnone (1997, 1999).  The logical conclusion to be drawn from these two sets of facts is that the ongoing rise in the air's CO2 content will help more of the extra organic matter that is produced under CO2-enriched conditions to remain in the soil even longer than it otherwise would remain due to the organic matter-conserving nature of the increased activities of the increased earthworm populations that have been shown to occur as a consequence of increases in the atmosphere's CO2 concentration.

References
Haynes, R.J. and Fraser, P.M.  1998.  A comparison of aggregate stability and biological activity in earthworm casts and uningested soil as affected by amendment with wheat and lucerne straw.  European Journal of Soil Science 49: 629-636.

Martin, A.  1991.  Short- and long-term effects of the endogenic earthworm Millsonia anomala (Omodeo) (Megascolecidae, Oligochaeta) of tropical savannas on soil organic matter.  Biol. Fertil. Soils 11: 234-238.

Zaller, J.G. and Arnone III, J.A.  1997.  Activity of surface-casting earthworms in a calcareous grassland under elevated atmospheric CO2.  Oecologia 111: 249-254.

Zaller, J.G. and Arnone III, J.A.  1999.  Interactions between plant species and earthworm casts in a calcareous grassland under elevated CO2.  Ecology 80: 873-881.