The sequestration potential of re-vegetated land - is the SOC stock the only relevant parameter?

Following a period of land degradation lasting more than one thousand years, Iceland has undertaken ambitious restoration and afforestation efforts over the last century. For some years, plantation activities have been financially supported by the Icelandic government due to the carbon sequestration potential of terrestrial carbon reservoirs to enhance the national carbon balance. To estimate the amount of carbon stored in the pedosphere of re-vegetated areas, most Icelandic studies have focused on the total soil organic carbon (SOC) stock without considering the quality of the SOC. Currently, little is known on the soil carbon dynamics at re-vegetated sites. This present study aims to characterize the sequestered carbon within different C-pools and determine its relative stability withthe purpose of assessing the long-term sequestration potential of soils at afforested sites. Differently aged restored mountain birch (Betula pubescens ssp. czerepanovii) woodlands (20, 30, 45 and 60 years) were compared to eroded and grassy lands and remnants of original mountain birch woodlands (old-growth) in southern Iceland. The soil profile was sampled at four different soil depths. Samples were treated by a physical and chemical fractionation procedure in order to determine C and N concentrations of various soil fractions. Further, stable N isotope ratios were used to characterize the relative stability of the carbon. Within the top 30 cm of soil,the total SOC stocks increase by age class (e.g. 20yrs: 31 ± 9; 60yrs: 45 ± 5 Mg ha -1 , old-growth: 61 ± 8 Mg ha -1 ), and confirm the findings from other studies undertaken at these sites. In the top 5 cm of the afforested sites, SOC is mainly stored as particulate organic material (POM), which contrasts with the old-growth site where C is equally distributed in POM and sandy mineral-associated material (mOM). Carbon within the silt and clay fractions becomes increasingly dominant in deeper soil layers in all age classes. A decrease of δ 15 N with increasing age (e.g. in the top 5 cm of soil, 20yrs: 0.53 ± 0.68; 60yrs; -2.44 ± 0.43 and old-growth: -3.46 ± 0.16) indicates a larger pool of fresh and less decomposed organic material which is rich in 14 N. This finding correlates well with the measured C:N ratios which increase with age class and decrease with increasing soil depth. This means that more stable SOC is predominantly stored within the deeper soil horizons. The study shows that SOC pools within afforested sites can consist of nearly 50 percent POM. This organic material will experience further decomposition, which will result in a lower sequestration potential of these sites than expected until now. This must be taken into account when calculating carbon budgets at national scales.