What was done
Open-top and -bottom chambers of 1.2-m² soil surface area were established in a species-rich grassland located near Basal, Switzerland, and fumigated continuously with atmospheric CO2 concentrations of 350 and 600 ppm, except during the winter months, for an entire year.  During the second year of differential CO2 treatment, earthworm surface casts were collected and weighed to determine the effects of elevated CO2 on earthworm activity.

What was learned
The mean annual soil moisture in the CO2-enriched chambers was 10% greater than that observed in the chambers receiving 350 ppm CO2.  Rates of earthworm surface cast production (g m^-2 day^-1), which were positively correlated with soil moisture, were up to six times greater in CO2-enriched chambers than they were in ambient control chambers.  This phenomenon contributed to a cumulative surface cast production value (g m^-2) that after only one year was 35% greater in the CO2-enriched chambers than in the control chambers maintained at the ambient CO2 concentration.  In addition, because earthworm casts are rich in organic carbon and nitrogen, the cumulative amount of these important nutrients on a per-land-area basis was 28% greater in the CO2-enriched chambers than it was in the ambient chambers.

What it means
As the CO2 concentration of the air continues to rise, it is likely that earthworms will increase their activities, which should promote soil processing and turnover.  In fact, the authors calculated that for current grassland ecosystems it would take earthworms approximately 100 years to egest a soil volume equivalent to that occupying the upper 15 cm of the soil profile.  In contrast, at an elevated CO2 concentration of 600 ppm, it would only take 75 years for earthworms to process an equivalent volume of soil.

As the atmospheric CO2 concentration increases, earthworm activities will likely accelerate plant litter decomposition and mineralization of soil organic matter, thereby increasing the total amount of plant-available nutrients within soils.  Thus, plant productivity and growth should increase even more in response to the rising CO2 content of the air than is implied by the direct aerial fertilization effect of elevated CO2, due to this indirect effect of atmospheric CO2 enrichment on enhancing soil fertility.