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Section 1.1.1: The use of equivalent quarz size and settling tube apparatus to fractionate soil aggregates by settling velocity
Book title
Geomorphological Techniques
Volume
1, section 1.1., 9 S.
Edition
Online Edition
Publisher
British Society for Geomorphology
Place of publication
London
ISSN/ISBN
2047-0371
Number
1.1
Keywords
settling tube apparatus, settling velocity, transport distance, aggregate fractionation
Abstract
In a given layer of surface runoff, particle transport distance declines with increasing settling velocity. Settling velocity itself is determined by the size, density and shape of the particles. For sediment composed of aggregates, settling velocity does not only vary due to texture, but also due to aggregation, aggregate size and stability. Therefore, aggregation can strongly affect the transport distance of the sediment and the substance specific redistribution of the eroded material, such as organic matter. Understanding the effect of aggregation, for example, on redistribution of eroded organic matter is therefore essential for understanding local, regional and global carbon cycles. To capture and establish the relationship between aggregation, settling velocity and aggregate specific organic matter content, a settling tube apparatus, based on a previous design, was constructed and applied to fractionate soils by water stable aggregate size classes. To illustrate the effect of aggregation on settling velocity, the results were compared with mineral grain sizes after ultrasound dispersion. Five settling velocity classes were distinguished based on the Equivalent Quartz Size (EQS) of particles ≥ 250 μm, 125 to 250 μm, 63 to 125 μm, 32 to 63 μm, and ≤ 32 μm. Fractionation of a silty loam by settling tube illustrates that aggregation strongly affects settling velocities and should be considered in erosion models, as opposed to the texture of mineral grains. An analysis of sediment organic matter in the five settling velocity classes also showed that settling velocity is a suitable parameter to physically connect the redistribution of eroded soil organic matter to overland flow transport processes.
