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 <63 µm fraction. However, after 50 years of birch growth on severely degraded soils the particulate organic matter (POM) fraction was significantly enriched most (+12 t POM-C ha⁻¹) in the top 30 cm. The study also found a doubling of the dissolved organic carbon (DOC) concentration after 50 years of birch growth. Therefore and due to the absence of any increase in the tested mineral-associated SOC fractions, we assume that the afforestation process evokes a carbon deposition in the labile SOC pools. Consequently, parts of this plant-derived, labile SOC may be partly released into the atmosphere during the process of stabilization with the mineral soil phases in the future. Our results are limited in their scope since the selected sites do not fully reflect the heterogeneity of landscape evolution and the range of soil degradation conditions. As an alternative, we suggest using repeated plot measurements instead of space-for-time substitution approaches for testing C changes in severely degraded volcanic soils. Our findings clearly show that detailed measurements on the SOC quality are needed to estimate the SOC sequestration potential of restoration activities on severely degraded volcanic soils, rather than only measuring SOC concentration and SOC stocks.

中文翻译：

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评估桦木造林后冰岛南部火山土壤的固碳潜力

绿化是一种在陆地系统中固存大气中的碳并增强生态系统服务的战略。因此，冰岛大片地区以前是植被繁茂的生态系统，现在已经退化。它们有很大的潜力充当碳汇。因此，这些景观系统的生态恢复是冰岛政府支持的减缓气候变化计划的一部分。这项研究的目的是探索在严重退化的土地上重建桦木林期间土壤有机碳（SOC）库的变化，并估计SOC封存潜力。比较了不同年龄的绿化山桦林站点（15、20、25和50年）与严重退化的土地的站点，自然生长的山桦林地和草地的残留物，在50年前使用化肥和草籽重新植被了。对土壤进行采样以估计SOC储量，并进行物理分馏以表征SOC的质量。我们的研究结果表明，严重退化的土壤可能会螯合额外的20 t C ha^-1（0–30 cm）可以到达自然生长的桦木林地的SOC储量。在桦树生长50年后，SOC储量明显低于自然生长的桦木林地，这表明绿化林分可以在生长50年后隔离更多的SOC。所述SOC分馏显示，在所有被测试的位点的碳的最被存储在<63  μ米级分。但是，在桦木在严重退化的土壤上生长了50年之后，颗粒有机物（POM）分数显着富集最多（ +12  t POM-C ha ^-1）放在顶部30厘米内。研究还发现，桦木生长50年后，溶解有机碳（DOC）浓度增加了一倍。因此，由于所测试的矿物相关SOC含量没有增加，我们假设绿化过程会在不稳定的SOC池中引起碳沉积。因此，这种植物衍生的，不稳定的SOC的一部分在将来与矿物土壤相稳定化的过程中可能会部分释放到大气中。由于选择的地点不能完全反映景观演化的异质性和土壤退化条件的范围，因此我们的研究结果范围有限。作为替代方案，我们建议使用重复绘图测量代替时空替代方法来测试严重退化的火山土壤中的碳变化。