Description and working steps
Add 30 grams of soil to 150 ml of deionized water and disperse ultrasonically with a power of 20 W and energy of 22 J ml-1. The ultrasonic power should be calibrated calorimetrically directly before the first application. If calibrated correctly, the time needed to achieve the output energy of 22 J ml-1 should be 177 seconds. The suspension is wet sieved over a 63-μm sieve until the rinsing water is clear. Use a minimum amount of 2000 ml of deionized water to flush the sample. Centrifuge the suspended fraction for 15 minutes at 2000 g (or the fastest speed that is possible) and then decant the supernatant. An aliquot of the decanted liquid is filtered through a 0.45-μm membrane filter and analysed for DOC. Dry both the sieving residue (>63 μm) and the centrifuged silt and clay fraction (<63 μm) at 40 °C and weigh it. Afterwards conduct a density fractionation with the fraction >63 μm by using a sodium polytungstate solution (about 50 ml) with a density of 2.0 g cm-3 . Centrifuge the tube for 15 minutes at 1000 g and transfer the floating POM fraction carefully into a sieve bag. The whole procedure should be repeated to ensure a complete isolation of the POM fraction before it is dried at 40 °C and weighed. Fill the remaining sand and stable aggregates fraction (S+A) into another sieve bag and wash both sieve bags with deionized water, dry at 40 °C and weigh. Homogenize the fraction <63 μm (s+c) gently with a mortar before a subsample of 1 g is used for oxidation with sodium hypochlorite (NaOCl): for this 50 ml 6% NaOCl, adjusted to pH 8 with concentrated HCl, is added to the subsample. After a reaction time of 18 hours at 25 °C the sample is centrifuged for 15 minutes at a 1000 g and washed with deionized water. The oxidation procedure should be repeated twice. Every time that fresh NaOCl is added to the sample, the soil should be mixed with a vortex mixer to ensure a complete reaction with the oxidizable C in the soil. Afterwards the subsample is dried at 40° C and weighed. Multiply the carbon in the one gram of sample with the weight of the s+c fraction to obtain the rSOC fraction.
This method was specifically designed to isolate fractions that are related to the model pools of the Rothamsted carbon model (RothC, Jenkinson & Rayner, 1977).
There is ample evidence that the obtained fractions do at least partly match RothC pools, particularly in agricultural soils.
High work load, technical requirements (centrifuge, ultrasound device), high costs for sodium polytungstate.
Zimmermann, M., Leifeld, J., Schmidt, M.W.I., Smith, P., Fuhrer, J., 2007. Measured soil organic matter fractions can be related to pools in the RothC model. European Journal of Soil Science 58, 658-667.
Poeplau, C., Don, A., Dondini, M., Leifeld, J., Nemo, R., Schumacher, J., Senapati, N., Wiesmeier, M., 2013. Reproducibility of a soil organic carbon fractionation method to derive RothC carbon pools. European Journal of Soil Science 64, 735-746.
Jenkinson, D.S. & Rayner, J.H., 1977. The turnover of soil organic matter in some of the Rothamsted classical experiments. Soil Science, 123, 298–305.