Nitrogen retention in soil organic matter (SOM) is a key process influencing the accumulation and loss of N in forest ecosystems, but the rates and mechanisms of inorganic N retention in soils are not well understood. The primary objectives of this study were to compare ammonium (NH4+), nitrite (NO2−), and nitrate (NO3−) immobilization among soils developed under different tree species in the Catskill Mountains of New York State, and to determine the relative roles of biotic or abiotic processes in soil N retention. A laboratory experiment was performed, where 15N was added as NH4+, NO2−, or NO3− to live and mercury-treated O horizon soils from three tree species (American beech, northern red oak, sugar maple), and 15N recoveries were determined in the SOM pool. Mercuric chloride was used to treat soils as this chemical inhibits microbial metabolism without significantly altering the chemistry of SOM. The recovery of 15N in SOM was almost always greater for NH4+ (mean 20%) and NO2− (47%) than for NO3− (10%). Ammonium immobilization occurred primarily by biotic processes, with mean recoveries in live soils increasing from 9% at 15 min to 53% after 28 days of incubation. The incorporation of NO2− into SOM occurred rapidly (<15 min) via abiotic processes. Abiotic immobilization of NO2− (mean recovery 58%) was significantly greater than abiotic immobilization of NH4+ (7%) or NO3− (7%). The incorporation of NO2− into SOM did not vary significantly among tree species, so this mechanism likely does not contribute to differences in soil NO3− dynamics among species. As over 30% of the 15NO2− label was recovered in SOM within 15 min in live soils, and the products of NO2− incorporation into SOM remained relatively stable throughout the 28-day incubation, our results suggest that NO2− incorporation into SOM may be an important mechanism of N retention in forest soils. The importance of NO2− immobilization for N retention in field soils, however, will depend on the competition between incorporation into SOM and nitrification for transiently available NO2−. Further research is required to determine the importance of this process in field environments.