I have not heard of earthworms suppressing nematode populations. They probably have some impact through incidental ingestion, but I don't know of any studies on targeted feeding of earthworms on nematodes as a protein source.
But...I do believe that earthworms are among the more sensitive organisms in the soil foodweb and that if they are present it is probably a pretty good indication that the foodweb is intact. In that case there should be an abundance of omnivore and carnivore species (nematodes, fungi, microarthropods) which may effect suppression of opportunistic plant-feeding nematode species.
I am skeptical that such suppression can be created by merely adding earthworms to soil. Rather, I think, it will involve managing and enhancing the foodweb by increasing the organic content and managing the soil with minimal disturbance and toxic shock (herbicides, mineral fertilizers, etc.). In that case, the appearance of earthworms would be an indicator of a structured (and perhaps suppressive) foodweb.
Many of the earthworms in soil are in the family Lumbricidae, class Oligochaeta, phylum Annelida. Other oligochaetes common in soil are the Enchytaeidae and the Megascolecidae.
Some keys are available in "Soil Biology Guide" by Daniel Dindal (John Wiley, 1990). Among the taxonomists I've heard of are Samuel James, Department of Biology, Maharishi International Univ., Fairfield, Iowa, and Donald Schwert, Geology Dept, North Dakota State Univ, Fargo, ND.
Soil biologists designate three groups of earthworms
Endogeic forms live and eat in and around the rhizosphere in mineral layers of soil. Probably ingesting decaying roots, fungi, bacteria and nematodes (of all trophic persuasions).
Epigeic forms live close to the soil surface associated with surface organic matter. Probably don't have much direct effect on rhizosphere organisms except maybe through mineralization of nutrients.
Anecic forms burrow directly down into the soil, come up at night and forage at the surface. Probably affect aeration and soil porosity, also redistribution of organic material through the soil profile.
For more information about nematodes, Go to Nemaplex Main Menu.