Organic matter characteristics in density fractionation of organic horizons with Asian dust and volcanic ash in snowy mountains of northern Japan

The process of organic horizon formation is reflected in the quantity and quality of the mineral materials contained. In the highland of mountains in northern Japan, thick organic horizons develop due to heavy snowfall. The organic horizons are rich in 2:1 clay minerals from Asian dust originating from the Eurasian Continent and active Al and Fe derived from volcanic ash. Organic matter in the organic horizons is present in separation or associations with these clay minerals or active Al and Fe. These organo-mineral associations make different biogeochemical properties between relatively free and mineral-associated organic matter in its dynamics, even in the organic horizons, because minerals can protect organic matter from microbial decomposition. Evaluating the biogeochemical arrangements of organic matter and minerals is valuable to understanding organic matter dynamics in the organic horizons containing rich minerals. The objective of this research is to evaluate the organo-mineral associations in the organic horizons in the snowy mountains of northern Japan using density fractionation.

The organic horizon samples were collected from three mountains. Two selected mountains (Mt. Chokai and Mt. Kurikoma) are volcanoes, and the other is a non-volcanic mountain (Mt. Makihata). Samples in Mt. Chokai were taken from each soil horizon from two soil profiles of snow meadow soils and one of dwarf bamboo and dwarf pine soil. In Mt. Kurikoma, one soil profile of a snow meadow was selected. One soil profile and two surface soils (5-15 cm) of snow meadow were collected in Mt. Makihata. Freeze-dried organic horizon samples were shaken with 1.6 g cm-3 SPT and grass beads at 16 h and 120 rpm. Recovered floating materials were sieved at 0.5 mm to remove coarse, fresh plant roots. The fraction larger than 0.5 mm was termed course light fraction (cLF), and the smaller fraction was termed small light fraction (sLF). The residue heavier than 1.6 g cm-3 was  heavy fraction (HF). Isolated fractions were freeze-dried and observed by SEM. Organic carbon and total nitrogen of isolated fractions were measured by an elemental analyzer.

The mass recovery ranged from 91.5 to 97.0%. A lower recovery rate was observed in the upper horizons, presumably due to losses attributable to higher dissolved organic carbon contents. Each isolated fraction separated by density and size showed different physicochemical properties. cLF mainly consisted of fresh roots. Decomposed plant residue was found in sLF. In HF, structures of highly decomposed plant residues combined with minerals were observed. Lower C/N of HF compared to cLF and sLF in Mt. Chokai indicated more decomposed organic matter associated with minerals. These results showed that even in organic horizons, organic matter had different physicochemical properties depending on their associations with minerals. This study highlights the importance of focusing on minerals in evaluating organic matter dynamics in organic horizons affected by aeolian dust. The stability of organic matter bound to minerals in organic horizons needs further evaluation.