Physical fractionation implies the isolation of SOC fractions along boundaries of physical properties, such as size, density or cohesive strength of aggregates. It emphasizes the importance of interactions between organic matter and mineral soil components, which are acknowledged to be crucial for SOC stabilization (Six et al., 2002; Eusterhues et al., 2003; Kaiser and Guggenberger, 2003). Physical fractionation methods can be divided into aggregate size fractionation, particle size fractionation and density fractionation. Thereby, the main difference between aggregate size fractionation and particle size fractionation is that the latter aims at a more or less complete dispersion of aggregates, while aggregate size fractionation methods aim at keeping the aggregates intact. Keeping the soil aggregates intact is useful for all research questions that specifically address soil structure, since it gives an idea about the current distribution of aggregates within the soil. Particularly fine micro-aggregates (<53 µm) are proposed to act as a physical barrier between decomposer and substrate, and thus protect SOC from rapid mineralization (Six et al., 2002). However, at the same time, the hierarchical structure of aggregates implies that a pure aggregate fractionation of SOC fractions with distinct turnover times likely fails, since larger aggregates contain smaller ones. Particle size fractionation after dispersion e.g. with ultrasonic, glass beads or hexametaphosphate (HMP), was shown to be more effective in isolating fractions with a wide range in turnover rates (Poeplau et al., under review).
Density fractionation is conducted for two different purposes: When only one density cut off is used, this is usually done to separate a light (fast turnover) from a heavy fraction (slow turnover). Common density cut offs are between 1.4 and 2 g cm-3 and separation is usually achieved by flotation and sedimentation in a dense solution (e.g. sodium polytungstate). This is done to separate the light fraction SOC that is presumably fresh and only loosely incorporated into the soil matrix from material bound to the heavier mineral phase. The free light fraction (isolated with no aggregate dispersion) as isolated by density fractionation is the youngest and most labile fraction. No other fraction, except for electrostatically separated free light fraction, was more enriched in young SOC (Poeplau et al., under review). Higher densities (up to 2.8 g cm-3) are applied to separate different minerals, which have been acknowledged for their different roles in SOC stabilization. Carbon fractions isolated with a density >2.8 g cm-3 are mainly bound to sesquioxides and were found to have extremely low turnover rates. In the following pages, a total of 12 different methods are presented that use physical fractionation methods.
Six, J., Conant, R., Paul, E.A., Paustian, K., 2002. Stabilization mechanisms of soil organic matter: implications for C-saturation of soils. Plant and Soil 241, 155-176. DOI: 10.1023/A:1016125726789
Eusterhues, K., Rumpel, C., Kleber, M., Kögel-Knabner, I., 2003. Stabilisation of soil organic matter by interactions with minerals as revealed by mineral dissolution and oxidative degradation. Organic Geochemistry 34, 1591-1600. DOI: 10.1016/j.orggeochem.2003.08.007
Kaiser, K., Guggenberger, G., 2003. Mineral surfaces and soil organic matter. European Journal of Soil Science 54, 219-236. DOI: 10.1046/j.1365-2389.2003.00544.x