Loss of plant biodiversity eliminates stimulatory effect of elevated CO₂ on earthworm activity in grasslands

Earthworms are among the world’s most important ecosystem engineers because of their effects on soil fertility and plant productivity. Their dependence on plants for carbon, however, means that any changes in plant community structure or function caused by rising atmospheric CO₂ or loss of plant species diversity could affect earthworm activity, which may feed back on plant communities. Production of surface casts measured during three consecutive years in field experimental plots (n = 24, 1.2 m²) planted with local calcareous grassland species that varied in plant species richness (diversity levels: high, 31 species; medium, 12; low, 5) and were exposed to ambient (356 μl CO₂ l⁻¹) or elevated (600 μl CO₂ l⁻¹) CO₂ was only consistently stimulated in high diversity plots exposed to elevated CO₂ (+120 %, 31 spp: 603 ± 52 under ambient CO₂ vs. 1,325 ± 204 g cast dwt. m⁻² year⁻¹ under elevated CO₂ in 1996; +77 %, 940 ± 44 vs. 1,663 ± 204 g cast dwt. m⁻² year⁻¹ in 1998). Reductions in plant diversity had little effect on cast production in ecosystems maintained at ambient CO₂, but the stimulatory effect of elevated CO₂ on cast production disappeared when plant species diversity was decreased to 12 and 5 species. High diversity plots were also the only communities that included plant species that an earlier field study showed to be among the most responsive to elevated CO₂ and to be most preferred by earthworms to deposit casts near. Further, the +87 % CO₂-induced increase in cast production measured over the 3 years corresponded to a parallel increase in cumulative total nitrogen of 5.7 g N m⁻² and would help explain the large stimulation of aboveground plant biomass production observed in high-diversity communities under elevated CO₂. The results of this study demonstrate how the loss of plant species from communities can alter responses of major soil heterotrophs and consequently ecosystem biogeochemistry.