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The influence of a combined elevation-vegetation site factor on the nature of the stable humus formed in soils derived from volcanic ash

University of British Columbia

An elevational sequence of soils on the volcano Iztaccihuatl was selected to study the effect of a combined elevation-vegetation site factor on the nature of the stable humus formed in soils developed from recent volcanic ash. One sample plot was selected in each of three vegetation zones; Abies religiosa, Pinus hartwegii and Zacatonal, an alpine grassland. In Part 1, non-parametric statistics were used to evaluate several parameters measured on the Ah₁ and Ah₂ soil horizons. In Part 2, composite samples were prepared and the polymeric humic acid and fraction A were bulk isolated. The results from Part 1 indicated that the soils were at an early stage in their development. The degree of mineral weathering increased with elevation and with depth in the profile. In the surface mineral horizons humus-Al complexes were dominant with insignificant formation of "amorphous" aluminosilicate clay minerals. Horizon differences in organic matter characteristics were not apparent in the Abies religiosa and Zacatonal zones, but were present in the Pinus hartwegii zone. The humus content differed little among the three zones, although qualitative differences related to the vegetation site factor were apparent in the kinds of organic components in the fulvic acid fraction. The Ch/Cf ratio 1% was related to the regional climate. E 1%/400 HA indicated that the humic acids from the three vegetation zones were similar in maturity; hence, zonal differences in E 1%/600 HA and E₄/E₆ HA were related to the average polymer molecular weight which increased from the Abies religiosa to the Zacatonal zone. In Part 2, elemental and infrared analysis failed to detect any zonal differences in the humic acid and fraction A. However, the data supported the rapid formation of a high molecular weight, highly aliphatic humic acid fraction rich in nitrogen. Four major phenolic acids of lignin origin were identified in the humic acid and fraction A hydrolysates -protocatechuic acid, p-hydroxybenzoic acid, vanillic acid and syringic acid - which established a lignin-derived component of the polymeric humus fractions. The source of vegetation for humus formation could be clearly distinguished from the ratios of the major phenolic acids identified.

Text

2010-06-20

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http://hdl.handle.net/2429/25870

Soil Science

University of British Columbia

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