Humus Composition of Charred Plant Residues

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Abstract
Japan is a typical volcanic country. Volcanic ash soils which develop in deposits of volcanic ash are widely distributed in Japan. Studies have shown that an important feature of humus in the A horizons of volcanic ash soils is the predominance of Type A humic acids with a high degree of humification (1, 2). The formation of Type A humic acids may proceed with the mediation of soil enzymes and inorganic components, since they are capable of accelerating the conversion of phenolic compounds to humic polymers with a high degree of darkening (3–5). As another possible mode, the authors suggested that the charring process of plants might contribute to the formation of Type A humic acids, because the abundant accumulation of humus in volcanic ash soils may be attributed to the presence of a dense grass vegetation and to the burning of these materials (6). Furthermore, carbonaceous materials such as charcoal and cinder are considered to be one of the possible sources of soil humus (7). In the present study, to validate the above hypothesis, the humus composition of three charred plant residues was analyzed, and their humic acids were compared with Type A soil humic acids. Susuki (Eulalia, Miscanthus sinensis A.), Sasa (Sasa, Sasa palmata N.), and Warabi (Bracken, Pteridium aquilinum K.) were used, as these plants are commonly growing on volcanic ash soils. The leaves and stems of the plants were crushed with a Wiley’s crusher and passed through a 2 mm sieve. To prepare the entirely charred residues, the beakers (200 ml) containing 10 g of each sieved material were placed in an oven at 250°C for 12 h. Humus composition of the charred plant residues obtained was analyzed according to the method described by KUMADA et al. (1). Humus was extracted by heating a mixture of the sample and 0.1 N NaOH solution for 30 min, and the humus extract was divided into humic and fulvic acids by adding cone. H2SO4. The precipitate, humic acid, was washed successively with H2SO4 (l : 100) and water, and then dissolved