Plantation forests are becoming increasingly important suppliers of fibre worldwide. In order to understand the consequences of converting natural forests to plantations, investigation of their ecological properties is warranted. We studied net N mineralization in a set of paired jack pine/black spruce or jack pine/aspen stands that included plantations and mature, natural stands in the Lake Nipigon area of northern Ontario. The general objective was to compare pairs of stands on similar soils but with different species composition. The buried bag technique was used to estimate net nitrogen mineralization rates from fall 2002 to spring 2004 in a total of 8 stands. Reported values pertain to the forest floor and top 15 cm of the mineral soil. In jack pine plantations aged 19 to 52, net N mineralization rates ranged from 16 to 51 kg•ha^-1•yr^-1 of NH₄⁺-N, and from -1 to 6 kg•ha^-1•yr^-1 of NO₃^--N. In black spruce plantations aged 17 to 41, from 39 to 42 kg•ha^-1•yr^-1 of NH₄⁺-N and from 3 to 6 kg•ha^-1•yr^-1 of NO₃^--N were produced. Paired mature, natural stands (~80-90 years old) of jack pine and black spruce had similar rates of net N mineralization overall, although NO₃^--N production was positive in the jack pine stand and negative in the black spruce. Rates of net N mineralization in a mature aspen stand were ~10 times higher than in an adjacent 52 year old jack pine plantation. The most pronounced differences in N mineralization rates among paired jack pine/black spruce stands were found in mid-aged plantations (41 years old). In general, rates of net NH₄⁺-N production were highest in midsummer, while net NO₃^--N production peaked in early fall. Peak production was usually followed by a period of net immobilization. The results suggest that species composition, as imposed by forest management, can affect nitrogen cycling processes.