Dust deposition influences properties of Icelandic birch woodland soils

Iceland has some of the world's most active dust sources. Volcanic activity shapes the nature of its ecosystems and soils, which mainly classify as Andisols. The dust deposited in Iceland consists of glass of basaltic origin, which releases Al, Fe, and Si during weathering. Areas close to dust hotspots, such as proglacial sites, can receive up to 1 kg/m² of dust per year. Naturally, these vast deposition rates influence soil composition and dynamics. In our study, we investigated the effects of dust deposition on soil properties in Icelandic birch woodlands, which are the only native woodlands. We selected ten study areas in old birch woodlands (60+ years old) throughout the country and classified each area according to its dust deposition rate, with 1 indicating very low (< 50 g/m²/yr) and 6 indicating extremely high deposition (500-1000 g/m²/yr). Soils were sampled to a depth of 30 cm. The birch woodland soils were all typical Andisols, with high carbon content and the presence of clay minerals, such as allophane and ferrihydrite, and metal-organic complexes. The results showed a great variability in the examined soil properties between areas of different dust categories, with clear trends. The carbon content and stocks in the top 30 cm were highest in areas far from dust sources and lowest in areas close to them. As the dust falls on the surface and slowly integrates with the soil, the carbon content in the soil dilutes; however, it also invokes carbon burial. We estimated that up to 26 g/m²/yr of carbon gets buried in areas close to dust hotspots (category 6). Thus, despite the carbon stocks being “low” in the top 30 cm of soil (~ 4 kg/m²), the overall carbon stocks across the entire soil profile may be greater than in areas with lower dust deposition rates due to carbon accumulation and burial. The same dilution effect was observed on clays and metal-organic complexes. Interestingly, the dust deposition correlated with the Al/Si ratio of allophane, with a lower Al/Si ratio in areas with higher dust deposition rates. These areas also had a higher soil pH and contained less active aluminium, which explains the lower allophane Al/Si ratios. All in all, dust deposition had a positive impact on the birch woodland soils, as it rejuvenates the soil, bringing fresh basaltic materials to the surface, raising the soil pH and CEC. We encourage further studies on dust deposition effects, especially in deeper soils (> 30 cm depth). This study highlights the importance of understanding the impact of dust deposition on soil dynamics, as it plays a crucial role in Icelandic ecosystems.