Soil formation and organic matter accretion in a young andesitic chronosequence at Mt. Shasta, California

Abstract

The objectives of this work were to study rates of increase in allophane concentration, specific surface area changes, and organic matter accretion in a young andesitic chronosequence. We sampled the 0–10-, 10–20-, 30–40-, 70–80-, and 140–150-cm depths of 77-, 255-, 616-, and approximately 1200+-year-old soils and analyzed them for allophane, ferrihydrite, specific surface area, cation exchange capacity, soil pH, and C and N concentrations. Allophane concentrations increased at rates up to a maximum of 0.14 g kg −1 year −1, and concentrations are up to 68 times higher in the oldest than in the youngest soil. During the same time ferrihydrite concentrations increased only by a factor of 2.3. The specific surface area that could be attributed to allophane was only 2–4% in the youngest soil but was 41–97% in the oldest soil. Carbon and N stocks increased linearly with soil age over the first ∼600 years with rates of 139 kg C ha −1 year −1 and 5.3 kg N ha −1 year −1, respectively. After about 600 years, accretion rates were lower. Increases in allophane concentrations lead to increased cation exchange capacity in the soil. Our results indicate that the ability of the soil to retain nutrients improved with soil development.