Aluminum-Humus Complexes for Accumulation of Organic Matter in Black-Colored Soils under Grass Vegetation in Bolivia

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Black-colored soils, equivalent to the Umbrepts suborder of USDA Soil Taxonomy, are distributed at altitudes ranging from 3,000 to 4,500m under grass vegetation in the mountainous area surrounding the Cochabamba basin of Bolivia. The soils had thick A horizons, as in the case of the Thick High-humic to Humic Andosols of Japan. The maximum value of organic C content of the A horizons was 157 g kg^<-1> but the predominant clay minerals were clay mica, its weathering products, and kaolinite. Attempts were made to analyze the characteristics of organic matter and the mechanism of its accumulation. Humic acids extracted by a 0.5% NaOH solution belonged to the A type for 10 samples including A1, A3, and C horizons collected at 3 sites, while those of the remaining two samples belonged to the B type. Amounts of Al extracted by the pyrophosphate solution ranged from 0.4 to 22.9 g kg^<-1> and was positively and significantly correlated with the organic C content. The high degree of humification of organic matter was ascribed to the grass vegetation and the burning of these materials. Formation of Al-humus complexes was considered to contribute to the accumulation and stabilization of organic matter. Chlorite could be a source of Al for the formation of complexes with humus.
社団法人日本土壌肥料学会の論文