Plantand Soil 187: 333-342, 1996. 333 @ 1996 KluwerAcademic Publishers. Printedin the Netherlands. Assessing the impact of elevated CO2 on soil microbial activity in a Mediterranean model ecosystem Shivcharn S. Dhillion l, Jacques Roy and Mary Abrams 2 Centre d'Ecologie Fonctionnelle et Evolutive, C.N.R.S., 1919 Route de Mende, BP 5051, F-34033 Montpellier, France. Present addresses: I SUM-University of Oslo and Department of Biology and Nature Conservation, Agricultural University of Norway (NLH), P O. Box 5014, As, N-1432, Norway* and 2BES, City of Portland, 1120 SW 5th Ave., Rm 400, Portland, OR 97204, USA Received 2 February 1996. Accepted in revised form 8 July 1996 Key words: CO2 enrichment, hyphal length, microbial biomass, microbial community, mycorrhizae, root length, root mass, soil enzymes, substrate utilization Abstract The fate, as well as the consequence for plant nutrition, of the additional carbon entering soil under elevated CO 2 is largely determined by the activity of soil microorganisms. However, most elevated CO2 studies have documented changes (generally increases) in microbial biomass and total infection by symbiotic organisms, which is only a first step in the understanding of the modification of soil processes. Using a Mediterranean model ecosystem, we complemented these variables by analyzing changes in enzymatic activities, hyphal lengths, and bacterial substrate assimilation, to tentatively identify the specific components affected under elevated CO2 and those which suggest changes in soil organic matter pools. We also investigated changes in the functional structures of arbuscular mycorrhizas. Most of the microbial variables assessed showed significant and substantial increase under elevated CO2, of the same order or less than those observed for root mass and length. The increase in dehydrogenase activity indicates that the larger biomass of microbes was accompanied by an increase in their activity. The increase in hyphal length (predominantly of saprophytic fungi), and xylanase, cellulase and phosphatase activities, suggests an overall stimulation of organic matter decomposition. The higher number of substrates utilized by microorganisms from the soil under elevated CO2 was significant for the amine/amide group. Total arbuscular and vesicular mycorrhizal infection of roots was higher under elevated CO2, but the proportion of functional structures was not modified. These insights into the CO2-induced changes in soil biological activity point towards potential areas of investigation complementary to a direct analysis of the soil organic matter pools. Introduction Increasing atmospheric CO2 has a number of direct and indirect effects on plants and their environments (e.g. Bazzaz, 1990; Rogers et al., 1994). Direct effects on soil of elevated CO2 have long been considered to be minimal, because of the high CO2 levels already present in most soils (Van Veen et al., 1991). Indirect effects via plants are thought to induce several major feedbacks on plants and to be critical determinants of ecosystem response to climate change (e.g. Diaz et al., FAX No: +4764948502. E-mail: shivcharn.dhillion @sum.uio.no @ibnf.nlh.no 1993; Norby, 1994; Rogers et al., 1994) but they are just starting to receive strong attention. In particular, an increasing number of studies point to below-ground systems as sinks in global C cycling (Norby et al., 1994; O'Neill, 1994). Microorganisms and their activities are the main mediators of C turnover in the soil. Microbial biomass regulated by substrate and water availability, tempera- ture, climatic regime and soil structure (Insam, 1990; Rice et al., 1994; Van Veen et al., 1991), is consid- ered a transformation agent of soil organic matter and a labile pool of organic and inorganic nutrients (Jenk- inson and Ladd, 1981). Therefore, its role in providing