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During the early establishment phase, outplanted white spruce (Picea glauca (Moench) Voss) and jack pine (Pinus banksiana Lamb.) seedlings are vulnerable to lethargic growth or mortality because of interspecific competition for soil nutrients, particularly N. Accurately quantifying the degree of N competition is essential for supporting effective vegetation management (VM) decisions. This study evaluated the use of in situ burials of ion-exchange membrane (IEM; Plant Root SimulatorTM-probes) for quantifying differences in soil N supply rate between different VM treatments and the relationship of this N availability index to early growth of conifer seedlings at four boreal forest sites. At most sites, the effect of noncrop N uptake on soil N availability was apparent, with smaller NH4+-N, NO3–-N, and total dissolved inorganic N (DIN) supply rates in control plots than in VM plots. Total DIN supply rate was correlated (R2 = 0.60 to 0.73, P < 0.01) with seedling height, root-collar diameter, and stem volume growth. Ammonium-N supply rate was better correlated than NO3–-N supply rate with conifer seedling growth, which is in agreement with preferential NH4+-N uptake by conifer species. The results of this study support the use of in situ burials of IEM for measuring soil N availability during the early establishment phase. [ABSTRACT FROM AUTHOR]