tElevated atmospheric CO2induced reductions in litter quality can adversely affect earthworms. However,this understanding is based on laboratory rather than field research and relates to single earthworm andtree species. Here earthworm populations were investigated under Alnus glutinosa, Betula pendula, andFagus sylvatica in a Free Air Carbon dioxide Enrichment field experiment. Litters from this experimentwere also fed to Lumbricus terrestris L. at two rates with live weight change and cast properties assessed.Elevated CO2(580 ppmv) reduced litter N (−12%) with a corresponding increase in C:N ratio, especiallyfor A. glutinosa. In the field, elevated CO2caused a shift in overall population composition, mainly char-acterised by reduced anecic biomass (–25%); endogeic and epigeic species were less affected. CO2effectson total biomass were most pronounced for A. glutinosa (e.g. field total biomass −47% vs. −11% overall).Growth of L. terrestris was lower when fed elevated CO2litter (−18%), although increased inputs of A.glutinosa litter mitigated this effect. In mesocosms, fresh cast respiration was lower (−14%) for elevatedCO2litter, an effect more pronounced for A. glutinosa (−24%). When normalised for C content, elevatedCO2effects on cast respiration were again negative and most marked for A. glutinosa litter. Litter N con-centration, and possibly ease of litter mineralisation were factors affecting litter resource quality LitterN and P concentrations varied with A. glutinosa > B. pendula > F. sylvatica; F. sylvatica had the highest cel-lulose content. Field earthworm biomass was higher under A. glutinosa compared with B. pendula and F.sylvatica (+17 and +70%, respectively); live weight increased with A. glutinosa litter in the feeding trialalmost three times more than for B. pendula, whereas it decreased for F. sylvatica. Cast respiration washighest for A. glutinosa, intermediate for B. pendula (ca. −36%) and lowest for F. sylvatica (ca. −78%). Earth-worm responses to elevated CO2were complex, being characteristic of individual tree and earthwormspecies; responses were more adverse for trees with higher quality litter and for anecic earthworms.