Boreal forest plantation growth depends on N supplied by net mineralization of organic N in the soil during the growing season. Logging slash quality and input can influence N availability after harvest on clearcut sites. Net production of organic and inorganic N and microbial biomass C and N contents were determined for forest floor and shallow mineral soil horizons from unharvested and clearcut boreal forest sites at the Esker Lakes Research Area (ELRA), in northeastern Ontario, Canada. Needle litter was added to unharvested forest floor material after 30 days during a 90-day laboratory incubation at constant temperature and moisture. Oe horizon material from 12-year old clearcut sites released 5 times more NO3- - N and twice as much inorganic N than soil collected from 2-year old clearcuts. This increase in NO3- - N accumulation was accompanied by decreases in both exchangeable NH4+ - N and microbial biomass C and N levels. Incorporation of black spruce needles into Oe material reduced the production of organic and inorganic N and increased microbial biomass N. Mineral soil release of inorganic N, while much lower than release from the forest floor, was greater in the 12-year old clearcut when compared to unharvested sites and 2-year old clearcut sites. Immobilization of N during decomposition of logging slash immediately after forest harvesting would reduce N losses from boreal forest cutovers. In the longer term, turnover of the microbial biomass formed during decomposition of slash may result in increased N availability to growing boreal forest plantations.