Background and Aims This study investigated initial land-use change effects on ecosystem biomass, carbon (C) and nitrogen (N) allocation and storage by comparing a recently afforested grassland with an adjacent intensively managed grassland in southern Ireland. Methods Soil C, N and fine root (<2 mm) stocks were determined from soil cores. Above ground biomass, C and N stocks were estimated from biomass clipping, inventory and allometric biomass equations developed for ash (Fraxinus excelsior L.) and black alder (Alnus glutinosa L.) in the 5-year-old forest plantation. Results Five years after grassland afforestation, the mean fine root stock of 0.31 kg m −2 in the forest was about half that of 0.64 kg m −2 in the grassland. This decrease was offset by an additional gain of 0.36 kg m −2 in tree biomass since afforestation. The above-to below ground biomass ratio shifted from 0.20 in the grassland to 1.59 in the forest. From May to October, mean net N mineralization was significantly lower in the forest compared to the grassland. Soil C and N concentrations in the 0-10 cm soil layer were significantly higher in the forest (62 mg C g −1 ; 5.7 mg N g −1 ) compared to the grassland (45 mg C g −1 ; 3.6 mg N g −1 ). However, the bulk density in the upper forest soil layer was lower than in the grassland. As a result, no differences existed between the respective total (0-30 cm depth) soil C and N stocks. Total ecosystem C and N storage was also similar for the forest (9.5 kg C m −2 ; 0.75 kg N m −2 ) and the grassland (9.3 kg C m −2 ; 0.77 kg N m −2 ). Conclusions A significant change in total ecosystem C and N following afforestation of this intensively managed grassland was not observed. Nevertheless, this study highlights immediate implications from such land-use change activities on biomass, C and N reallocation among the above-and belowground ecosystem pools which may subsequently affect ecosystem biogeochemical cycles.