森林土壌における有機物の存在形態(その2)

抄録

Humus forms in each layer of forest soils were examined for organic matter composition, humus composition, elementary composition and nitrogen transformation using brown forest soils in Chubu district of Japan. The results obtained are summarized as follows. 1) The L, F and H layers and A horizons of several forest soils were fractionated physically. The L layers were mainly composed of Lf_1 (fresh litter) or Lf_2 (decomposed to some extent, > 2mm) and Lf_3 (decomposed, > 2mm). Fifty to ninety percent of the F layers were Ff_1 (> 2 mm) or Ef_2 (1-2 mm), crushed organic particle fractions. Ffty to eighty percent of the H layers was Hf_6 (< 0.1 mm, precipitate after dispersing in water overnight). A large portion of plant remains in the A horizons originated from roots at various stages of decay. 2) The physical fractions of each layer of the A0 horizon were analysed for the organic matter composition by the modified WAKSMAN's method. The amount of lignin and protein increased with the progress of decomposition of organic matter in A_0 horizons, while the total amounts of each extract benzeneethanol extract and hot water extract, except for the HCl extract, and the amounts of sugar and starch, phenolic substances, hemicellulose and pectin, and cellulose decreased. Sugars and starch comprised only a small portion of the hot water extract, and polyphenolic substances comparable to sugars and starch were also found in the extract. Hemicellulose and pectin accounted for only about one-half of the HCl extract. 3) The physical fractions of each layer and horizon were examined for humus composition by the KUMADA's method. Most of the humic acids from plant residual fractions of A_0 horizons belonged to the Rp type. The humic acids form the L layers showed a tannin-like character which disappeared in the F layers, and humic acids from the fine particle fractions of the H layers contained Pg fractions. The humic acids from A horizon belonged to the P or B type and contained Pg fractions. 4) On the changes in elementary composition with decomposition and humification of organic matter, the change in C/N ratio was the most obvious compared to the others. The decomposition process of organic matter within A_0 horizons can be divided into two stages based on the changes in C and N content, the first stage from the L to F or F-H layer and the second from the F or F-H to H or H-A layer. The C/N ratios of mineral horizons were lower in moderately moist type (B_D) than in dry type (B_A, B_B) soils. It was specially pointed out that Hf_7 and Hf_8, fine particle and water-soluble fractions from H layers, respectively, were fairly different materials from the other fractions, and were suspected to play an important role in soil formation. 5) The samples of each layer and horizon from the 29 brown forest soils were examined for nitrogen mineralization by determining the inorganic nitrogen during 20 week's incubation at 30℃. The higher the NT (total nitrogen) level of the sample, the greater the content of N_I (total inorganic nitrogen) was. Both the N_I level and the nitrogen mineralization rates (N_I/N_T×100) showed a tendency to increase with decreasing the C/N ratio from the L layer to the F or F-H layer, and thereafter decreased with increasing depth. Most of the N_I existed as NH_4-N in the majority of the samples except that remarkable nitrification was observed in the F, F-H, H, H-A layers and A horizon from some moderately moist type soil with the vegetation of Cryptomeris japonica or brood-leaf tree. In the samples where much NO_3-N was generated, ammonification and nitrification took place alternately. The mineral horizons of the moderately moist type soils were distinguished from those of the dry type soils by their higher NI levels and higher values of the NO_3-N/N_I ratio.