Minimisation of organic matter disruption during particle-size fractionation of grassland epipedons

In order to be able to avoid artefacts during fractionation of soil into primary particles, knowledge is required about the soil organic matter (SOM) behaviour when dispersing soil aggregates from different environments. This study was designed to investigate dispersion behaviour of macro- and microaggregates of native grassland epipedons from different climatic regions. Samples were collected from eight grassland epipedons to a depth of 10 cm along a climosequence in the prairie from Central Saskatchewan, Canada, to Southern Texas, USA. The samples were sieved to 2 mm and ultrasonically dispersed at 0, 1, 5, 15, 25, and 75 kJ, with partial dispersion also at 2 and 3 kJ. Carbon and nitrogen analysis was performed on the <20 mu m, 20-250 mu m and 250-2000 mu m size separates of all dispersion treatments. Dispersion of macroaggregates (250-2000 mu m) was achieved at 1 kJ ultrasonic energy for most of the sites whereas 3 kJ were needed for dispersing macroaggregates at wet extremes in the prairie. More ultrasonic energy (greater than or equal to 5 kJ) was required to disperse microaggregates (20-250 mu m). Intense dispersion (>5 kJ), however, disrupted particulate organic matter. Ultrasonic dispersion at 75 kJ was different for different ultrasonic instruments, suggesting that the calorimetric calibration was not very suitable for standardisation of ultrasonic energy input. For minimisation of artefacts, it is suggested that particulate SOM should be removed after weak dispersion (greater than or equal to 3 kJ), and, prior to subsequent aggregate disruption at 22 kJ, input energy should be checked at intervals by ultrasonic pressure measurements to determine whether the tip of the probe has deteriorated. (C) 1999 Elsevier Science B.V. All rights reserved.