Enhanced microbial activity in the rhizosphere of tree roots may represent a mechanism by which tree species increase nutrient availability in infertile soils. I hypothesized that rhizosphere effects on microbial activity would be suppressed in fertile soils, and that such effects would be greater for ectomycorrhizal (EM) tree species than arbuscular mycorrhizal (AM) tree species. At the Turkey Hill Plantations (THP) in Dryden, NY, replicate plots of sugar maple (Acer saccharum M.), an AM tree species, and northern red oak (Quercus rubra L.), an EM tree species, were fertilized from 2001-2003. At the Hubbard Brook Experimental Forest (HBEF), W. Thornton, NH, replicate plots of sugar maple and yellow birch (Betula allenghaniensis B.), an EM tree species, were also fertilized from 2001-2003. In 2003, rhizosphere and bulk soils were sampled from the organic horizon of control and fertilized plots at both sites. In general, fertilization suppressed rhizosphere effects on microbial activity in both EM tree species. In red oak plots, enhancement of microbial biomass in the rhizosphere of control plots (20-31%) was significantly smaller than in the fertilized plots (5-7%). Similarly, the rhizosphere effect on C mineralization (12%) and phosphatase enzyme activity (26%) observed in control plots was not apparent in fertilized plots. At the HBEF, rhizosphere effects on C mineralization and phosphatase activity in control plots (20% and 22%, respectively) were greater than in fertilized plots (1% and -8%, respectively). In plots of both EM tree species, fertilization significantly reduced mycorrhizal colonization which suggests that decreased C flux to mycorrhizae may have mediated rhizosphere effects. At both sites, fertilization resulted in small but non-significant reductions in rhizosphere effects in sugar maple. My results suggest that rhizosphere effects in some tree species may be suppressed by increases in nutrient availability, possibly as a result of decreased C allocation to mycorrhizae.