Soil carbon stocks of regenerating Icelandic native birch woodlands: Effects of space and time.

Icelandic native ecosystems and soils have been severely degraded since settlement in the 9th century. Today, barren landscapes occupy about 45 % of the land surface and only 1.5 % is covered by native birch woodlands versus 20-40 % in pre-settlement times. Iceland's soils are mainly Andisols, among the most carbon-rich soil orders owing to their unique colloidal characteristics. Hence, there is tremendous potential to sequester soil carbon in degraded soils through revegetation activities. The restoration of birch woodlands is considered a national priority, which may significantly impact the nation's carbon budget. The objective of this study was to determine soil carbon concentrations and stocks across chronosequences (0 to 60+ years) of birch woodlands under diverse geographical conditions comprised of ten study areas across Iceland. The highest carbon stocks were found in old birch woodlands with a mean of 7.4 kg C m^-2 in the top 30 cm soils, which is unusually high compared to other Nordic deciduous woodlands. We attribute this to andic soil properties that effectively stabilize and sequester soil organic matter. Calculated soil carbon accumulation rates were 0.01 kg m^-2 yr^-1 for the first 30 years of birch woodland establishment and 0.04-0.07 kg m^-2 yr^-1 in mature woodlands (30-60 years old). These accumulation rates, if applied to large-scale birch woodland restoration plans, would amount to 20 % of the current total CO₂ emissions of Iceland (not counting LULUCF). Importantly, we found a significant impact of dust deposition (up to 1 mm yr^-1) on soil carbon stocks, contributing to carbon burial (∼26 g m^-2 yr^-1) in areas close to dust hotspots. Birch restoration further stabilizes soils from erosion and the above-ground biomass serves as an efficient dust collector. This study documents the potential of birch restoration as a highly effective strategy to address soil degradation and promote soil carbon sequestration across Iceland.