The fate and effects of nano copper oxide (nCuO) to soil invertebrates in field collected soil and in biosolid amended field soil were investigated in order to understand the risk CuO nanomaterials present to the soil environment. The effects of nCuO were compared to a soluble Cu-salt (i.e., CuSO_⁴) through sub-lethal toxicity studies with the earthworm Eisenia andrei and the springtail Folsomia candida, which included survival and reproduction tests for both species and bioaccumulation in E. andrei. The fate of both Cu forms was examined in soil through the characterization of Cu, namely total Cu, extractable Cu²⁺ and extractable nano particulate CuO in each exposure scenario. In general, exposure to nCuO compared to CuSO₄ was less toxic for both reproduction and bioaccumulation tests and with or without biosolid amendment. Results demonstrated that the solubilization of and availability of Cu²⁺ in soils could explain the observed effects.