AimsGovernment policy in Ireland is to increase the national forest cover from the current 10% to 18% of the total land area by 2020. This represents a major land use change that is expected to impact on the national carbon (C) stocks. While the C stocks of ecosystem biomass and soils of Irish grasslands and coniferous forests have been quantified, little work has been done to assess the impact of broadleaf afforestation on C stocks.MethodsIn this study, we sampled a chronosequence of ash (Fraxinus excelsior) forests aged 12, 20, 27, 40 and 47 years on brown earth soils. A grassland site, representative of the pre-afforestation land use, was sampled as a control.Important FindingsOur results show that there was a significant decline (P < 0.05) in the carbon density of the soil (0-30 cm) following afforestation from the grassland (90.2 Mg C ha(-1)) to the 27-year-old forest (66.7 Mg C ha(-1)). Subsequently, the forest soils switched from being a C source to a C sink and began to sequester C to 71.3 Mg C ha(-1) at the 47-year-old forest. We found the amount of C stored in the above- and belowground biomass increased with age of the forest stands and offset the amount of C lost from the soil. The amount of C stored in the above-and belowground biomass increased on average by 1.83 Mg C ha(-1) year(-1). The increased storage of C in the biomass led to an increase in the total ecosystem C, from 90.2 Mg C ha(-1) at the grassland site to 162.6 Mg C ha(-1) at the 47-year-old forest. On a national scale, projected rates of ash afforestation to the year 2020 may cause a loss of 290 752 Mg C from the soil compared to 2 525 936 Mg C sequestered into the tree biomass. The effects of harvesting and reforestation may further modify the development of ecosystem C stocks over an entire ash rotation.