Global Change Biology (1997) 3, 473–478 Soil fungal-arthropod responses to Populus tremuloides grown under enriched atmospheric CO 2 under field conditions JOHN N. KLIRONOMOS,* MATTHIAS C. RILLIG,† MICHAEL F. ALLEN,† DONALD R. ZAK,‡ MARK KUBISKE§ and KURT S. PREGITZER§ *Department of Botany, University of Guelph, Guelph, Ontario, Canada N1G 2W1, † Department of Biology, San Diego State University, San Diego, CA 92182 USA, ‡School of Natural Resources and Environment, University of Michigan, Ann Arbor, MI 48109 USA, §School of Forestry and Wood Products, Michigan Technological University, Houghton, MI 49931 USA Abstract We investigated the influence of elevated CO 2 and soil N availability on the growth of arbuscular mycorrhizal and non-mycorrhizal fungi, and on the number of mycophagous soil microarthropods associated with the roots of Populus tremuloides. CO 2 concentration did not significantly affect percentage infection of Populus roots by mycorrhizal or non-mycorrhizal fungi. However, the extra-radical hyphal network was altered both qualitatively and quantitatively, and there was a strong interaction between CO 2 and soil N availability. Under N-poor soil conditions, elevated CO 2 stimulated hyphal length by arbuscular mycorrhizal fungi, but depressed growth by non-mycorrhizal fungi. There was no CO 2 effect at high N availability. High N availability stimulated growth by opportunistic saprobic/pathogenic fungi. Soil mites were not affected by any treatment, but collembolan numbers were positively correlated with the increase in non-mycorrhizal fungi. Results indicate a strong interaction between CO 2 concentration and soil N availability on mycorrhizal functioning and on fungal-based soil food webs. Keywords: arbuscular mycorrhiza, global change, Glomales, microarthropod, soil fungi Received 24 September 1996; revision accepted 3 December 1996 Introduction Rhizosphere fungi are important regulators of plant productivity and of nutrient cycles in terrestrial eco- systems (Allen 1991). They are a diverse and ubiquitous set of heterotrophic microorganisms which can function as decomposers, pathogens, parasites, and mutualistic symbionts (Kendrick 1992), and as such they can provide both positive and negative feedback effects on plant growth. It is hypothesized that rhizosphere fungi will be greatly affected by an increasing atmospheric CO 2 concentration (Allen et al. 1995), since elevated CO 2 increases carbon allocation below-ground in the form of increased root growth and rhizodeposition (Zak et al. 1993; Rogers et al. 1994). As the soil microbial community is typically carbon-limited (Zak et al. 1994), an increase in substrate is expected to increase fungal activity and/ or abundance. Also, since fungi are the main food source for many common soil invertebrates (Visser 1985) it is Correspondence: John N. Klironomos, fax +1 519 767 1991, e-mail jklirono@uoguelph.ca © 1997 Blackwell Science Ltd. 473 expected that elevated CO 2 will stimulate carbon cycling in below-ground food webs. Various fungal groups do not exist independently in soil and when considering the enormous functional diversity within the fungal kingdom (Kendrick 1992), extrapolating behaviour of one fungus to the entire kingdom is misleading. For example, in a recent pot study, plant-mediated changes resulting from elevated atmospheric CO 2 did not affect total microbial biomass, but dramatically altered the balance between mycorrhizal and non-mycorrhizal fungi and bacteria (Klironomos et al. 1996). Although rhizosphere fungi have been previously studied in response to atmospheric CO 2 fertilization (see O’Neill 1994), most studies have focused on specific types of organisms under controlled conditions (either mycorrhizal symbionts or specific plant pathogens). Alternatively, total microbial biomass has been reported (Zak et al. 1993), but this measure lacks enough resolution to make any conclusions at the functional level. Functionally and structurally, rhizosphere fungi can be