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Aggregate hierarchy in soils
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1991
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The study fractionated Alfisol, Mollisol, and Oxisol soils into particle-size classes using a series of gentle to vigorous disaggregation treatments. Alfisols and Mollisols exhibited a stepwise breakdown from macroaggregates (>250 µm) to microaggregates (20–250 µm) and finally to <20 µm particles, whereas the Oxisol remained stable to rapid wetting but released <20 µm particles only after vigorous disruption and showed no aggregate hierarchy, SEM revealed roots and hyphae as stabilizing agents for larger aggregates, suggesting that organic matter drives hierarchy in Alfisols and Mollisols but oxide dominance in Oxisols suppresses it.
An Alfisol, a Mollisol and an Oxisol were fractionated into different particle sizes after a range of disaggregating treatments from gentle to vigorous. The Alfisol and the Mollisol appeared to break down in steps; macroaggregates >250 �m diameter breaking down to microaggregates 20-250 �m diameter before particles <20 �m were released. Vigorous disruption led to particle size distributions similar to those obtained by classical methods used to determine particle size distributions. The Oxisol was stable to rapid wetting treatments but when aggregate disruption was initiated by vigorous treatments particles <20 �m diameter were released and there was no evidence of aggregate hierarchy. Scanning electron microscopy of particles of different sizes showed distinctly single grain particles and aggregates. The microscopic studies indicated the potential role of roots and hyphae in the stabilization of larger aggregates, and for fragments of roots as nuclei for smaller aggregates. Plant debris was not visible in aggregates <20 �m but clay microstructure was evident. It is suggested that aggregate hierarchy occurs in Alfisols and Mollisols because organic materials are the dominant stabilizing agents in larger aggregates but in the Oxisol oxides are dominant stabilizing agents and prevent the expression of aggregate hierarchy caused by organic materials.