To understand the impact of rising levels of atmospheric CO2 on ecosystems, we need to understand plant responses to elevated CO2, as well as how those plant responses in turn affect their environment. An important component of the environment of a plant is the soil biota living near plant roots. Soil nematodes are representative of a large portion of this biota, since they are abundant and trophically diverse in most soils. In a three-year field experiment, we studied the responses of soil nematodes to increased root growth of trees growing in high and low nitrogen soils under ambient and twice-ambient atmospheric CO2, a two-by-two factorial experimental design. Our hypothesis was that in the high-N soil, increased root growth resulting from twice-ambient atmospheric CO2 would positively affect nematode density, supporting a more abundant and trophically complex nematode community. Trembling aspen (Populus tremuloides) were grown in twenty open-top chambers under the four treatments, replicated five times. In low-N soil, twice-ambient CO2 was associated with higher density of the most abundant plant-feeding taxon (Trichodoridae), lower density of one bacteria-feeding taxon (Rhabditidae), and lower evenness of the community, compared to ambient CO2. In high-N soil, twice-ambient CO2 was associated with higher density of predator/omnivores, lower diversity, and a larger value of Bonger’s Maturity Index, compared to ambient CO2. In soils under young deciduous trees, such as the aspens in this experiment, increased root growth under elevated CO2 may result in significant changes in soil food web community structure that may provide clues about the fate of carbon under elevated CO2.