Air-drying causes an alteration in density fractionation of soil materials from organic horizons containing volcanic ash

Density fractionation is commonly used to isolate organic matter in soils based on organo-mineral associations. Not only mineral horizons but also organic horizons contain mineral components. Soil samples for density fractionation are sometimes air-dried or freeze-dried, which can introduce unexpected biases in the results. In this study, we compared the density fraction distributions in wet, air-dried, and freeze-dried organic horizon samples rich in reactive Al and Fe minerals. On Mt. Chokai, a volcano in northern Japan, samples were collected from organic horizons from three representative vegetation types: K1 (dwarf pine and dwarf bamboo at the summit), K2 (dwarf bamboo on the back slope), and K3 (snow meadow with predominantly Poaceae at the foot slope). Wet, air-dried, and freeze-dried samples were subjected to sequential density fractionation using cutoff values of 1.6 and 1.8 g cm–3. We then determined the mass, organic carbon, and total nitrogen distributions in the isolated fractions. Air-drying altered the mass, organic carbon, and total nitrogen distribution of each density fraction. Although we observed differences in the mass distributions of the freeze-dried and wet samples in the snow meadow samples (K3), the mass distributions in the K1 and K2 samples did not change significantly between the freeze-dried and wet samples. Therefore, freeze-drying is recommended as a pretreatment to minimize the effects of drying on density fractionation, particularly for soil samples with no drying history, high organic matter content, and/or high levels of reactive Al and Fe minerals.