Soil dynamics within Icelandic birch woodland chronosequences

Abstract

The reduction of natural woodland cover in the world has led to a significant deterioration in soil fertility and disruption of soil nutrient cycles. In Iceland, land degradation increased vastly after the erasure of mountain birch woodlands, the country’s only native forest-forming tree species. The Icelandic government aims to restore part of these lost woodlands. This thesis seeks to fill important knowledge gaps regarding birch woodland soils in relation to woodland restoration. Ten birch woodland chronosequences (non-forested to 60+ years old woodlands) across the country were investigated, with a focus on soil organic carbon (SOC), soil chemical and colloidal constituents, and hydrological processes. The soils are typical Andisols with a colloidal fraction dominated by allophane, ferrihydrite, and mineral-organic complexes (MOCs). The research revealed that allophane in the soils had extremely variable Al/Si ratios, which were related to influences of soil age, dust accumulation rates, and soil pH. The results showed significant SOC sequestration in old birch woodland soils, with carbon stocks of 7.4 kg/m2 in the top 30 cm (significantly higher than non-forested sites: 5.0 kg/m2), and a yearly SOC accumulation of 0.04-0.07 kg/m2/yr. The rapid increase was mostly attributed to andic soil properties and bonds with soil colloids, including MOCs, which enhanced SOC stability. Ferrihydrite was more abundant in the soils than was expected and had an influence on carbon dynamics. The restoration of birch woodlands to 5% of the land cover would lead to a SOC sequestration totaling 7% of the country’s current greenhouse gas emissions. The water retention capacities of the old woodlands were also the highest in this study (FC 63.5%, WP 45.6% on average), with excessively rapid infiltration rates (>586 mm/h in summer). This can be explained by the site’s porous soils and rich vegetation cover, which assisted the water inflow and prevented ice blockage during winter (a key factor for hydrological soil health). Dust emerged as an influential factor on soil properties, such as carbon dynamics and Al/Si ratios of allophane, and by buffering the soil pH. Dust deposition buries carbon (up to 0.026 kg/m2/yr in the highest dust categories), contributing to SOC accumulation. In general, natural birch woodlands in Iceland positively impact soil quality, as their soils are carbon-rich, fertile systems, with optimal hydrological properties. Their conservation and restoration are of fundamental environmental value in Iceland today and for Iceland of tomorrow.