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a Environmental Engineering Section, Dep. of Life Sciences, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
b Dep. of Agroecology, Danish Institute of Agricultural Sciences, P.O. Box 50, DK-8830 Tjele, Denmark
* Corresponding author (C.Kjaergaard{at}agrsci.dk).
Received 3 July 2003.
The fraction of clay that disperses in water, water-dispersible clay (WDC), is recognized as an important property with respect to predicting soil erosion and colloid leaching. Using six mineralogically similar soils with 12, 18, 24, 28, 37, and 43% clay, we studied the influence of soil clay content, initial matric potential (IMP; 
= –2.5, –100, and –15500 hPa), and wetting rate on WDC released in response to infiltration of low–ionic strength rainwater, using a low-energy input measurement of WDC (LE-WDC). These results were referenced by WDC obtained by a conventional, high-energy input measurement based on air-dried soil (HE-WDC). The energy input in the dispersion procedure significantly affected the release of WDC. The amount of HE-WDC increased with clay content, while the amount of LE-WDC decreased with increasing clay content. The decrease in LE-WDC was explained by an increase in cohesive strength, reflected by the increase in water-stable aggregates (
4 mm). A strong dependency of IMP on LE-WDC was observed, with maximum release of LE-WDC from soils that were at –2.5 hPa before measurement. Decreasing soil matric potential in the period before measurement reduced LE-WDC and also reduced the dependency of soil clay content, with soils incubated at –15500 hPa releasing a low amount of LE-WDC independent of clay content. The content of particulate organic C (POC) in the LE-WDC decreased with increasing clay content, and increased after drying to –15500 hPa. Colloid dispersibility changed as a function of time and moisture status, with the main changes occurring during or immediately after adjustment of the moisture content. Increasing the wetting rate resulted in a doubling of the amount of LE-WDC released from the initially dry soil (–15500 hPa), while no effect of wetting rate was observed at higher initial matric potentials.
Abbreviations: CEC, cation exchange capacity • DOC, dissolved organic C • EC, electrical conductivity • HE-WDC, high-energy water-dispersible colloids • IMP, initial matric potential • LE-WDC, low-energy water-dispersible colloids • POC, particulate organic C • SAR, sodium adsorption ratio • TOC, total organic C • WDC, water-dispersible colloids • WSA, water-stable aggregates
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