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Ferro-humic podzols of coastal British Columbia : aspects of genesis and chemistry Sanborn, Paul Thomas


Three related aspects of the genesis and chemistry of Ferro-Humic Podzols in coastal British Columbia were investigated: micromorphology, phosphorus forms, and biological aluminum cycling. Profile morphologies display horizon disruption by slope instability and windthrow, with a patchy distribution of organic matter in the solum. Maximum C concentrations often occur at the base of the B horizon above a root-restricting layer. Channel-like features formed by root growth and decomposition provide important hydrologic pathways and may be either strongly enriched in organic matter (> 17% C) and Al, or partially eluviated. Microfabrics of surface organic and eluvial horizons resemble those of other forest Podzols. Organic matter-rich zones within B horizons have distinctive fabrics consisting of amorphous organic materials and/or humified, but recognizable, root detritus. Few living roots occur, suggesting that these features may be relict. Root residues appear to serve as deposition sites of Al and other metals. Such features appear to form through both in situ decomposition and illuviation, with little influence by faunal activity. Bhf horizon fabrics may be dominated by granular aggregates of presumed faunal origin, although with finer textures, microstructures tend to be more blocky. In coarser-textured B horizons, fabrics tend to be dominated by coatings and infillings of organic materials, often with a high proportion of recognizable plant tissue residues. Extraction data suggest the occurrence of allophanic materials in some B horizons with C contents exceeding 15%, with such cases associated with high Fe concentrations. Complex yellow and red colour banding in void infillings may indicate compositional zonation. A sequential phosphorus extraction and fractionation technique indicated a degree of weathering of primary phosphates comparable to that in Ultisols. Although total P was greatly depleted from eluvial horizons, organic P (Po) was dominant throughout the sola of eight profiles. Bicarbonateextractable Po, thought to be the most labile Po fraction, was more abundant than in many Mollisol A horizons. Correlations suggested a role for this fraction in supplying labile inorganic P in B horizons. Levels of resistant residual P in two regional groups of B horizons were directly related to Fe content. Al, organic matter, and Po levels are directly related in B horizons, suggesting a common illuvial origin. Annual return of Al in needle litter (approximately 1 kg ha⁻¹) was similar in both western hemlock and Douglas-fir stands. In fresh hemlock needles, much of the Al content is easily leached by dilute neutral salt solutions; comminution increases this extractability. During decomposition, the proportion of pyrophosphate-extractable Al increases, suggesting a shift to organic-complexed forms. Biologically-cycled forms may comprise much of the Al flux in the upper solum, with litter comminution by soil fauna partially controlling the rate of release during decomposition.

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