摘要：Abstract. Afforestation is a strategy to sequester atmosphericcarbon in the terrestrial system and to enhance ecosystem services.Iceland's large areas of formerly vegetated and now degraded ecosystemstherefore have a high potential to act as carbon sinks. Consequently, theecological restoration of these landscape systems is part of climatemitigation programmes supported by the Icelandic government. The aim of thisstudy was to explore the change in the soil organic carbon (SOC) pools andto estimate the SOC sequestration potential during the re-establishment ofbirch forest on severely degraded land. Differently aged afforested mountainbirch sites (15, 20, 25 and 50 years) were compared to sites of severelydegraded land, naturally growing remnants of mountain birch woodland andgrasslands which were re-vegetated using fertilizer and grass seeds 50 yearsago. The soil was sampled to estimate the SOC stocks and for physicalfractionation to characterize the quality of the SOC. The results of ourstudy show that the severely degraded soils can potentially sequester anadditional 20 t C ha−1 (0–30 cm) to reach the SOC stock of naturallygrowing birch woodlands. After 50 years of birch growth, the SOC stock issignificantly lower than that of a naturally growing birch woodland,suggesting that afforested stands could sequester additional SOC beyond 50years of growth. The SOC fractionation revealed that at all the tested sitesmost of the carbon was stored in the µm fraction.However, after 50 years of birch growth on severely degraded soils theparticulate organic matter (POM) fraction was significantly enriched most(+12 t POM-C ha−1) in the top 30 cm. The study also found a doublingof the dissolved organic carbon (DOC) concentration after 50 years of birchgrowth. Therefore and due to the absence of any increase in the testedmineral-associated SOC fractions, we assume that the afforestation processevokes a carbon deposition in the labile SOC pools. Consequently, parts ofthis plant-derived, labile SOC may be partly released into the atmosphereduring the process of stabilization with the mineral soil phases in thefuture. Our results are limited in their scope since the selected sites donot fully reflect the heterogeneity of landscape evolution and the range ofsoil degradation conditions. As an alternative, we suggest using repeatedplot measurements instead of space-for-time substitution approaches fortesting C changes in severely degraded volcanic soils. Our findings clearlyshow that detailed measurements on the SOC quality are needed to estimatethe SOC sequestration potential of restoration activities on severelydegraded volcanic soils, rather than only measuring SOC concentration andSOC stocks.
