ABSTRACT: Effects of logging on imogolite-type material in Podzols of the Sunshine Coast and implications for water quality.

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Imogolite-type material refers to poorly crystalline aluminosilicates with an Al:Si ratio of 2:1. It is comprised of (1) imogolite, a mineral of stoichiometry (OH)3Al2O3SiOH, and (2) proto-imogolite allophane, a less ordered material with similar composition. Imogolite-type material is a naturally occurring pedogenic mineral. It was first identified in volcanic soils and is also a common phase in illuvial horizons of podzols. Because of its poorly crystalline nature, ITM is among the most reactive components of soil and can be readily impacted by environmental disturbance. It exhibits a large specific surface area and bears a weak permanent positive surface charge, and is an important mineral for the anion retention capacity of many volcanic soils and podzols. Imogolite-type material also releases significant amounts of Al ions upon dissolution and may be involved in the occurrence of Al toxicity in drainage waters often reported after logging on acidic forest soils. The objectives of this research are to: (1) document the nature and amount of poorly ordered aluminosilicates, including ITM, at the study sites, (2) determine the impact of forest harvesting on ITM, and (3) describe the effects of ITM content variations on soil properties and water quality. The study area is located in the Roberts Creek Study Forest (Sunshine Coast, British Columbia, Canada). Soils are Humo Ferric Podzols in the Canadian system of soil classification. We collected soil samples from undisturbed forested plots (control), and compared them to samples from plots that were clear-cut harvested 1 to 8 years ago. Samples were analyzed for poorly ordered aluminosilicates using selective dissolution analysis and transmission electron microscopy. Other analyses conducted for site characterization included soil pH, soil organic matter and organic C content, dissolved organic matter concentration, exchangeable cations and anions, plant-available and total phosphorus and nitrogen, soil solution composition, and texture. Selective dissolution of poorly ordered aluminosilicates revealed that the atomic Al:Si ratio equals 2, indicating the presence of ITM. Transmission electron micrographs showed no sign of imogolite morphology (tubes or threads) but instead showed aggregates of spherical nanoparticles consistent with the presence of proto-imogolite allophane. Proto-imogolite allophane is believed to be the main form of poorly ordered aluminosilicates in the Roberts Creek podzols. Control sites are very high in ITM, containing up to 40g/kg of ITM in the Bf, BC, and upper C horizons. Imogolite-type material content decreases significantly after logging to a low of 5g/kg occurring in the 8 years old harvested plot. There is no sign of recovery of the ITM pool after 8 years following logging. Additional sampling was conducted in August 2005 to include plots that were logged up to 15 years ago.

Bibliography of Canadian Geomorphology