This study investigated differences in the magnitude and partitioning of the carbon (C) and greenhouse gas (GHG) balances in an age sequence of four white pine (<i>Pinus strobus</i> L.) afforestation stands (7, 20, 35 and 70 years old as of 2009) in southern Ontario, Canada. The 4-year (2004–2008) mean annual carbon dioxide (CO<sub>2</sub>) exchanges, based on biometric and eddy covariance data, were combined with the 2-year means of static chamber measurements of methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) fluxes (2006–2007) and dissolved organic carbon (DOC) export below 1 m soil depth (2004–2005). The total ecosystem C pool increased with age from 46 to 197 t C ha<sup>−1</sup> across the four stands. Rates of organic matter cycling (i.e. litterfall and decomposition) were similar among the three older stands. In contrast, considerable differences related to stand age and site quality were observed in the magnitude and partitioning of individual CO<sub>2</sub> fluxes, showing a peak in production and respiration rates in the middle-age (20-year-old) stand growing on fertile post-agricultural soil. The DOC export accounted for 10% of net ecosystem production (NEP) at the 7-year-old stand but <2% at the three older stands. The GHG balance from the combined exchanges of CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O was 2.6, 21.6, 13.5 and 4.8 t CO<sub>2</sub> equivalent ha<sup>−1</sup> year<sup>−1</sup> for the 7-, 20-, 35- and 70-year-old stands, respectively. The maximum annual contribution from the combined exchanges of CH<sub>4</sub> and N<sub>2</sub>O to the GHG balance was 13 and 8% in the 7- and 70-year-old stands, respectively, but <1% in the two highly productive middle-age (20- and 35-year-old) stands. Averaged over the entire age sequence, the CO<sub>2</sub> exchange was the main driver of the GHG balance in these forests. The cumulative CO<sub>2</sub> sequestration over the 70 years was estimated at 129 t C and 297 t C ha<sup>−1</sup> year<sup>−1</sup> for stands growing on low- and high-productivity sites, respectively. This study highlights the importance of accounting for age and site quality effects on forest C and GHG balances. It further demonstrates a large potential for net C sequestration and climate benefits gained through afforestation of marginal agricultural and fallow lands in temperate regions.
