The objective of this study was to evaluate the impact of pre-commercial thinning in the boreal forest on soil fertility and soil microbial biomass in three soil moisture regimes: xeric mesic, and humid. The treatment consisted of thinning balsam fir (Abies balsamea) stands to an average densisty of 3,000 stems/ha. Soil temperature was highter in the thinned plots and the availability of potassium, phosphorus and base saturation also increased. Initial levels of inorganic nitrogen before incubation were higher following thinning, however net mineralization tended to decrease. The increase in microbial biomass-N confirmed that the treatment contributes to N immobilization in the short term. The thinning treatment also increased concentrations of N, P, and K in current 1-year and 2-year old needles.