Humus characteristics of Andisols were studied to determine the genesis of melanic epipedons in the xeric soil moisture regime of northern California. The humus horizons of the eight pedons used in this study showed a very dark color characteristic of melanic epipedons even though they were formed from forest vegetation. According to Keys to Soil Taxonomy, the humus horizons of seven pedons satisfied the criteria for melanic epipedons. The humic acid fraction extracted by sodium hydroxide was dominated by A-type humic acids exhibiting the highest degree of humification. Data from this study and the literature indicate that andic soil properties contribute to the accumulation of large concentrations of humus. It was also observed that the large accumulation of organic matter directly leads to the formation of melanic epipedons rather than fulvic Andisols. Analysis of the soil samples for opal phytoliths showed that the influence of gramineous vegetation during soil formation was negligible. The presence of large concentrations of carbonaceous fragments in the soil samples suggests that fire has been an important factor in the formation of melanic epipedons in the xeric moisture regime of northern California.
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