Afforestation is a strategy to sequester atmospheric carbon in the terrestrial system and to enhance ecosystem services. Iceland's large areas of formerly vegetated and now degraded ecosystems therefore have a high potential to act as carbon sinks. Consequently, the ecological restoration of these landscape systems is part of climate mitigation programmes supported by the Icelandic government. The aim of this study was to explore the change in the soil organic carbon (SOC) pools and to estimate the SOC sequestration potential during the re-establishment of birch forest on severely degraded land. Differently aged afforested mountain birch sites (15, 20, 25 and 50 years) were compared to sites of severely degraded land, naturally growing remnants of mountain birch woodland and grasslands which were re-vegetated using fertilizer and grass seeds 50 years ago. The soil was sampled to estimate the SOC stocks and for physical fractionation to characterize the quality of the SOC. The results of our study show that the severely degraded soils can potentially sequester an additional 20 t C ha−1 (0–30 cm) to reach the SOC stock of naturally growing birch woodlands. After 50 years of birch growth, the SOC stock is significantly lower than that of a naturally growing birch woodland, suggesting that afforested stands could sequester additional SOC beyond 50 years of growth. The SOC fractionation revealed that at all the tested sites most of the carbon was stored in the
This work contributes to the CarbBirch project funded by Orkuveita Reykjavikur and the work within the Nordic Centre of Advanced Research on Environmental Services (CAR-ES) and the Forest Soil C-sink Nordic Network (FSC-Sink). We want to thank our lab technician and friend Marianne Caroni, who sadly left us much too early, for her help and inspired discussions. We would also like to extend our gratitude to Ruth Strunk and Judith Kobler for their help in the laboratory during carbon and volcanic clay measurements. Nina Carle and Mathias Würsch helped during data gathering in the field and in the laboratory. Our sincerest thanks go to Gudmundur Halldorsson and the people of the Soil Conservation Service at Gunnersholt for their help and hospitality. Further, the authors gratefully acknowledge Vladimir Wingate for improving the grammar. The comments provided by Lorenzo Menichetti, Robert Qualls and Steven Sleutel are much appreciated.