Description and working steps
The fractionation scheme is based on the method by Golchin et al. (1994), but is modified in a way that the dispersion step to release occluded light fraction (oLF) is omitted. Thus, bulk soil (<2mm) was only separated into light fraction (LF) and heavy fraction (HF) by density fractionation using sodium polytungstate (SPT) at a density of 1.6 g cm-3. The selection of density cut-off at 1.6 g cm-3 is based on recommendations by Cerli et al. (2012), who could show that it was the most suitable density to separate maximum carbon content in light fractions and to minimize contamination of light fractions with floating soil minerals.
Sodium polytungstate (SPT 0, TC Tungsten Compounds, Grub am Forst, Germany) solution was adjusted to a density of 1.6 g cm-3 by weighing out defined volumes of the solution. It should be noted that during the whole procedure, SPT solutions should be collected, since there is the possibility to recycle them for reuse with the method described by Six et al. (1999). Dried and sieved (<2 mm) bulk soil samples were suspended in SPT solution (soil : solution ratio, 1 : 5; 8g soil : 40ml SPT solution) in centrifuge tubes (approx. total volume 50 ml, 27 mm inner diameter, 115 mm height, conical bottom) and subsequently centrifuged at 4500 g for 10 min. LF material was collected by careful removal of the floating material and subsequent filtration using glass fiber filters (GF 6, Whatman, Dassel, Germany). LF material was rinsed with deionized water until the electrical conductivity was <50 µS cm-1, to remove residual SPT. The remaining soil material with a density >1.6 g cm-3 (HF) was washed three times with deionized water following centrifugation each time. All fractions can be either oven- or freeze-dried, depending on further analyses to be done on the fractions. Soil samples were fractionated in triplicate and weights were recorded to determine recovery of the fractionation procedure.
The scheme is a simplification of the Golchin method which omits the sonication step to release the occluded light fraction (since this fraction is often very small in mass and chemically not very distinct from the other fractions).
The scheme is intended to separate physically unprotected particulate organic matter (light fraction, LF) from other soil organic matter which can be associated with soil aggregates or minerals (heavy fraction, HF). However, the density solution (SPT) itself has a dispersing effect and could contribute to the breakup of soil aggregates and release of organic matter therein (depending on the energy which is needed to break the aggregates, which varies for different soils).
Simple and fast.
The heavy fraction is not separated any further and is therefore a heterogeneous SOC pool.
Sollins, P., Spycher, G., Glassman, C., 1984. Net nitrogen mineralization from light-and heavy-fraction forest soil organic matter. Soil Biology and Biochemistry 16, 31-37.
Cerli, C., Celi, L., Kalbitz, K., Guggenberger, G., Kaiser, K., 2012. Separation of light and heavy organic matter fractions in soil — Testing for proper density cut-off and dispersion level. Geoderma 170, 403–416.
Golchin, A., Oades, J., Skjemstad, J., Clarke, P., 1994. Study of free and occluded particulate organic matter in soils by solid state 13C CP/MAS NMR spectroscopy and scanning electron microscopy. Australian Journal of Soil Research 32, 285–309.
Six, J., Schultz, P.A., Jastrow, J.D., Merckx, R., 1999. Recycling of sodium polytungstate used in soil organic matter studies. Soil Biology and Biochemistry 31, 1193–1196.