Anaerobic ammonium oxidation with nitrite reduction to dinitrogen (termed anammox) has been reported to be an important process for removing fixed nitrogen (N) in marine ecosystems and in some agricultural and wetland soils. However, its importance in upland forest soils has never been quantified. In this study, we evaluated the occurrence of anammox activity in two temperate forest soils collected from northeastern China. With 15N-labeled NO3− incubation, we found that the combined potential of the N2 production rates of anammox and codenitrification ranged from 0.01 ± 0.01 to 1.2 ± 0.18 nmol N per gram of soil per hour, contributing 0.5% to 14.4% of the total N2 production along the soil profile. Denitrification was the main pathway of N2 production and accounted for 85.6% to 99.5% of the total N2 production. Further labeling experiments with 15NH4+ and 15NO2− indicated that codenitrification was present in the mixed forest soil. Codenitrification and anammox accounted for 2% to 12% and 1% to 7% of the total N2 production, respectively. Two anammox species, “Candidatus Brocadia fulgida” and “Candidatus Jettenia asiatica,” were detected in this study but in very low abundance (as indicated by the hzsB gene). Our results demonstrated that the anammox process occurs in forest soils, but the contribution to N2 loss might be low in these ecosystems. More research is necessary to determine the activities of different N2 releasing pathways in different forest soils.