Description and working steps
The method has been developed in order to gain insight in the different stabilizing mechanisms of soil organic matter. It intends to differentiate between organic matter originating from plant residues in different stages of degradation versus organic matter associated with minerals in the fine fraction. Free particulate organic matter (POM) is obtained after density separation. Therefore 30 g of air-dried and sieved (< 2 mm) soil are mixed with 300 mL of sodium polytungstate solution with a density of 1.8 g cm-3. Subsequently two fractions of POM of different size (< 20 µm and > 20 µm) are obtained after disruption of soil aggregation by ultrasonication with 450 J mL−1 and thus are considered to be occluded POM now liberated from soil aggregates. In contrast to particle-size separates which also isolate the POM fractions in the sand-sized fractions (Oades, 1988), the use of a density separation allows to obtain all these POM fractions in a concentrated form and thus allows further characterization of these POM fractions, e.g., by solid-state 13C NMR spectroscopy. The remaining mineral-associated fraction is separated again at 20 µm to gain a small and a large mineral fraction, because this is also the threshold value used in the estimation of soil C saturation in the approach by Hassink et al. (1997).
Steffens, M., Kölbl, A., Kögel-Knabner, I., 2009. Alteration of soil organic matter pools and aggregation in semi-arid steppe topsoils as driven by organic matter input. European Journal of Soil Science 60, 198-212.
Oades, J.M., 1988. The retention of organic matter in soils. Biogeochemistry 5, 35-70.
Hassink, J., 1997. The capacity of soils to preserve organic C and N by their association with clay and silt particles. Plant Soil 191: 77–87.