Relationships Among pH, Minerals, and Carbon in Soils from Tundra to Boreal Forest Across Alaska

Tundra and boreal forests in northern high latitudes contain significant amounts of carbon (C) in the soil, indicating the importance of clarifying controls on soil C dynamics in the region and their feedback effects on climate systems. In northern Alaska, variations in soil C processes are closely associated with variations in soil acidity within ecosystems; however, the reason for this association remains unclear. In this study, we demonstrate that it results from weathering and subsequent changes in soil geochemical characteristics, including minerals and adsorptive organic C. We sampled soils from 12 sites in Alaska along a 600-km transect from the Arctic Ocean to interior Alaska, spanning the biomes of tundra, tundra-boreal forest ecotone, and boreal forest. Mineral soil analyses revealed that soils with low pH have fewer base cations, more aluminum/iron minerals, and lower base saturation, indicating that weathering is a major function of these geochemical characteristics in the broad area over northern Alaska. Adsorbed organic C in soil presented strong correlations with Al and Fe minerals, soil pH, and soil total C and represented approximately 30-55% of total soil C, suggesting that soil C accumulation in the Alaskan ecosystems is strongly controlled by weathering-related changes in geochemical characteristics. An adsorption test supported these observations and illustrated a greater capacity for acidic soil to adsorb organic C. These findings demonstrate that variations in weathering-associated characteristics have a strong influence on the regional variation in C dynamics and biogeochemical consequences in the Alaskan ecosystems.