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Effects of Elevated CO2 and Earthworm Casts on Grassland Plant Species
Reference
Zaller, J.G. and Arnone, J.A., III.  1999.  Interactions between plant species and earthworm casts in a calcareous grassland under elevated CO2Ecology 80: 873-881.

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 nearly one-and-a-half years.  As part of this project, the authors investigated whether or not plant species (graminoids, non-legume forbs, and legumes) growing near earthworm surface casts were more responsive to CO2 enrichment than those growing further away from such nutrient-rich microsites.

What was learned
At ambient CO2 concentration, the association of plant species with earthworm casts was highly variable, and remained that way under conditions of atmospheric CO2 enrichment as well.  Generally, plants growing in close proximity to earthworm casts produced more biomass than similar plants growing further away from such casts, regardless of CO2 concentration.  When assessing the influence of earthworm casts on plant responsiveness to atmospheric CO2 enrichment, however, no statistically significant results were detected.  Nonetheless, the average growth response of graminoids to elevated CO2 was greater for those species that were growing closer to earthworm casts, in comparison to those that were growing further away from them.  Thus, it is conceivable that plant growth responses to atmospheric CO2 enrichment can be increased if local plant niches are closely associated with earthworm casts, which provide limiting nutrients to facilitate greater plant growth.

What it means
As the atmospheric CO2 concentration continues to rise, nearly all plants will exhibit increased photosynthetic rates and enhanced biomass production both above- and belowground.  This increase in root production, along with enhanced nutrient exudation from roots, should increase the organic matter content of nearly all soils.  In turn, greater soil organic matter contents will stimulate many earthworm activities, including the egestion of nutrient-rich casts that increase the fertility of soils.  And with greater soil fertility, the absolute growth response of plants to rising levels of atmospheric CO2 should increase.  Thus, this beneficial circular relationship between earthworms and plants, which is mediated by soil organic matter content, is increasingly-enhanced by the rising CO2 content of the air.  And this phenomenon makes the soils a better place for both plants and earthworms.


Reviewed 1 December 1999