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Chemical properties of root mats in some south coastal British Columbia podzols : implications for classification and genesis

University of British Columbia

Podzols, situated at high elevation within the Coast Mountains surrounding Vancouver, often have accumulations of organic material within the mineral soil either as mats above an impermeable layer or as channels running obliquely and horizontally through the B horizon. This organic material is "greasy" to the touch with few discernible mineral grains on hand texturing. The mats range in thickness from 1 cm to greater than 40 cm, are situated in seepage zones and are overlain by a well drained sandy loam, often of low organic content. The channels have the gross morphology of roots and are up to 5 cm wide. Occasional remnants of wood structure are discernable in situ and the cellular nature of the organic material can be seen under the compound microscope. The majority of the mats could be classified as organic (>17% C), either as an H or an O horizon. If they contained less than 17% C they met the criteria of a Bhf horizon. Chemically the mats and channels have some unusual properties. They are similar to the surficial humus in carbon to nitrogen ratio and nitrogen to sulphur ratio but much closer to a podzolic B horizon in iron content. The ratio of carbon in humic acids to fulvic acids (Ch/Cf) and polyphenolic to total fulvic acids (Ca/Cf) was also more characteristic of a podzol. The organic mats had an average Ch/Cf of 0.23 and a Ca/Cf of 0.73. This is in comparison with 2.44 and 0.44, respectively, for the surficial humus and 0.34 and 0.72, respectively, in the organic material of the conventionally situated Bhf horizons. Both mats and channels had very high concentrations of aluminum, sometimes exceeding 10%. From the organic mats on Burke Mountain, aluminum extracted by acid ammonium oxalate was just slightly higher than that extracted with sodium pyrophosphate and oxalate silicon was low. This implied that most of the aluminum was bound with the organic material. In contrast, the oxalate aluminum of Cypress Park mat samples was often more than double that which was extracted with pyrophosphate and oxalate silicon values were also high. The figures indicated that allophane, with an approximate AltSi ratio of 2, may make up a high proportion of this horizon. Morphologically, the mats and channels seem to have originated through root decomposition but chemically, podzolic mechanisms seem to have played an important role. Because of similarities between soils studied in this thesis and soils of the literature found to contain imogolite, a column study was undertaken to test a mechanism whereby decomposing roots could be enriched by the aluminum of proto-imogolite. It was found that the organic material of rotting wood could scavenge aluminum from this positively charged aluminosilicate colloid and that in the process the fulvic acids of the wood were also immobilized. If a mechanism similar to this was operating within a podzol, it could help explain both the high concentration of aluminum and of fulvic acids in the mats and channels.

Text

2010-06-27

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http://hdl.handle.net/2429/26008

Soil Science

University of British Columbia

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