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.