Fractionation procedure. The size fractionation procedure is based on the method of Jocteur Monrozier et al. (27) and was performed with sieved samples (#2 mm) as described in detail by Stemmer et al. (51). Essentially, the soil-water
suspension was dispersed by low-energy sonication (output energy of 0.2 kJ g21) and subsequently fractionated by a combination of wet sieving and repeated centrifugation to avoid disruption of microaggregates. Particle size fractions of
200 to 63 mm (fine sand fraction), 63 to 2 mm (silt fraction), and 2 to 0.1 mm (clay fraction) were obtained without the addition of a flocculant. The fraction samples were freeze-dried.