A Climosequence of Soils Occurred in Kali Gandaki Valley in Central Nepal. : 2. Soil Organic Matter Accumulation and Humus Composition.

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Other Title
  • 中央ネパール・カリガンダキ河流域に分布する土壌の気候的遷移について : II.土壌有機物の蓄積と腐植組成
  • 中央ネパール・カリガンダキ河流域に分布する土壌の気候的遷移について-2-土壌有機物の蓄積と腐植組成
  • チュウオウ ネパール カリガンダキガワ リュウイキ ニ ブンプスル ドジョウ

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Abstract

This study was carried out for the purpose of elucidating a sequence of soil organic matter using evidences obtained on organic matter accumulation and humus composition in Kali Gandaki valley located in central Nepal. The area is divided into six different climatic regions : humid subtropical, humid temperate, subhumid temperate, semiarid temperate and arid temperate from the north to the south, and subalpine with elevations ranging from 3000 to 4000 m. The results obtained showed a considerably clear transition in some properties of soil organic matter caused by the climatic differences. 1) The accumulation of organic carbon in mineral soil horizons was highest (180-219 ton/ha) in the humid temperate region and decreased in the order of humid temperate ≧ subhumid temperate (165 ton/ha, except for the AC soil) > subalpine (110-173 ton/ha) ≧ semiarid temperate (75-146 ton/ha) ≧ humid subtropical (69-116 ton/ha) > arid temperate (16-79 ton/ha). Changes in the total nitrogen accumulation as influenced by the climate were basically similar to those of the organic carbon accumulation. The ratio of the accumulated amount of C to N in each soil profile varied from 10 to 18, and was higher (16-18) in the semiarid or arid temperate region because of the slower rate of organic matter decomposition caused by the aridity. 2) The ratio of extracted humic fractions (humic and fulvic acids) to the accumulated organic carbon in each soil profile (C_E/C_T) ranged mostly between 50 to 52% and was slightly lower in the humid subtropical zone (43.8%, except for No. 2) and in the semiarid temperate zone (47%}. This indicates that the extraction of humic fraction is more difficult in highly humified and relatively fresh organic matter than in moderately decomposed one. The Ch/Cf ratio showed a tendency to decrease with depth in most soil profiles. The ratio of accumulated humic acid to fulvic acid in each profile was highest (1.39-1.65) in the subhumid temperate followed by the semiarid temperate region (0.94-1.03), ranging between 0.35 to 0.84 in the soils belonging to the other climatic regions. The significant correlation obtained between the Ch/Cf ratio and the NH_4OAc (pH 7)-extractable Ca in A or A_1 horizon suggested that the formation of humic acid is accelerated when the exchangeable Ca content of the soil is high. The types of humic acid obtained from samples, except for those in the 0-5 or 0-10 cm horizons, were Rp, P-B, P-A, and A in the arid to semiarid temperate, subhumid temperate and subalpine, humid temperate and humid subtropical, respectively. It is obvious that the wamer and the more humid the climatic condition of the region in which the soil is present, the higher is the degree of humification of the humic acid. This may suggest the intensity of soil organic matter decomposition by micro-organisms is closely related to the humification of humic acids. The Pg (green fraction of humic acid) content was higher in the deeper parts of most soil profiles, and was higher in the subhumid temperate (400-470 kg/ha) and the subalpine (184-439 kg/ha) than in the other regions. The alkaline to weak alkaline soils of the arid to semiarid temperate regions contained no Pg fraction. The conditions for Pg production are presumed to exist in acidic soils in the humid temperate region.

Journal

  • Pedologist

    Pedologist 29 (1), 18-32, 1985-06-30

    Japanese Society of Pedology

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