decomposition and nutrient mineralization, leading to a reduced host dependency on the mycorrhizal symbiosis. In scenario C, the change in leaf litter chemistry (enhanced nitrogen content) increases resource quality, and the mycorrhizal fungi have to compete with the saprotrophic community for access to mineralized nutrients by increasing diversity and extraradical hyphal exploitation of soil. Source: Adapted from Dighton and Jansen (1991).

The effect of enriched atmospheric CO2 on soil organisms is inconsistent (Kandeler et al., 1998; Bardgett et al., 1999). Increased carbon dioxide and elevated temperature increased microbial carbon, but decreased the metabolic quotient when temperature alone was increased. The increase in CO2, however, led to an increase in root biomass and an increase in the C:N ratio, possibly because of a change in the balance between allocation of carbon to root growth and carbon storage. The inconsistency in the pattern of belowground response to elevated CO2 was echoed by Zak et al. (2000), who summarized the results of 47 publications on soil C and N cycling under elevated carbon dioxide. The basic generalities of these studies, spanning graminoid, herbaceous, and woody plant ecosystems, were that (1) there was greater plant growth under elevated CO2, with more

carbon entering the below-ground component, and (2) that there was greater metabolic activity of soil microbial communities under elevated CO2. Changes in C and N cycling between life forms were significant, leading to coefficients of variation between 80800%. The increase in plant biomass also corresponds to an increase in soil organic matter accumulation in temperature grassland ecosystems (Hunt et al., 1991), but the increased storage was not enough to keep pace with the rate of CO2 buildup in the atmosphere.