Comparing Model Projections with Observations: Worms
Small oligochaete worms known as enchytraeids, according to Maraldo et al. (2010), “are widely distributed from the Arctic to tropical areas, and typically inhabit the organic horizon in soils,” where they “contribute to the decomposition processes and nutrient mineralization.” These activities have been shown to lead to increased nutrient availability and uptake by plants (Laakso and Setala, 1999; Cragg and Bardgett, 2001). Enchytraeids provide these benefits directly, as these authors describe it, “by consuming large amounts of organic matter,” and indirectly “by their feeding activity and modifications of soil structure.” And they note, in this regard, “the presence of enchytraeids is especially important in nutrient poor ecosystems” such as “temperate heathland and northern coniferous forests, where their biomass dominates the soil faunal community,” citing the work of Cragg (1961) and Swift et al. (1998).
Working on a hilly nutrient-poor sandy soil with a dry heath/grassland cover at Brandbjerg, Denmark, the seven scientists conducted an experiment beginning October 2005 and extending through 2007. They studied the individual and combined effects of (1) soil warming: a mean daily temperature increase of 0.3°C in winter and 0.7°C in summer at a depth of 5 cm, provided by a scaffolding that carried a curtain—which reflected the outgoing infrared radiation from the soil/plant surface back toward the ground—that was automatically pulled over the vegetation at sunset and retracted at sunrise; (2) drought: peak soil water content reductions of 11 percent and 13 percent compared to control plots in 2006 and 2007, provided by waterproof curtains that were automatically pulled over the vegetation during rain events; and (3) atmospheric CO2 enrichment: a CO2 concentration increase from 382 to 481 ppm, provided by a free-air CO2 enrichment (FACE) system.
Maraldo et al. report their experimentally imposed warming had no significant impact on enchytraeid biomass production, but their drought treatment decreased it by 40 percent. On the other hand, the extra 99 ppm of CO2 stimulated enchytraeid biomass by 40 percent. They remark that at certain times this latter phenomenon was “especially positive,” as in the summer of 2007, when they state “the total enchytraeid biomass in the CO2 plots was increased by 108% compared to ambient plots.” They found no interactions among the three factors, so “the positive effect of increased CO2 [+40%] and the negative effect of drought [-40%] were cancelled out when applied in combination.”
Cragg, J.B. 1961. Some aspects of the ecology of moorland animals. Journal of Ecology 49: 477.
Cragg, R.G. and Bardgett, R.D. 2001. How changes in soil faunal diversity and composition within a trophic group influence decomposition processes. Soil Biology and Biochemistry 33: 2073–2081.
Laakso, J. and Setala, H. 1999. Sensitivity of primary production to changes in the architecture of belowground food webs. Oikos 87: 58–64.
Maraldo, K., Krogh, P.H., van der Linden, L., Christensen, B., Mikkelsen, T.N., Beier, C., and Holmstrup, M. 2010. The counteracting effects of atmospheric CO2 concentrations and drought episodes: studies of enchytraeid communities in a dry heathland. Soil Biology & Biochemistry 42: 1958–1966.
Swift, M.J., Andren, O., Brussaard, L., Briones, M., Couteaux, M.M., Ekschmitt, K., Kjoller, A., Loiseau, P., and Smith, P. 1998. Global change, soil biodiversity, and nitrogen cycling in terrestrial ecosystems—three case studies. Global Change Biology 4: 729–743.