Description and working steps
Thirty grams of air-dried soil were mixed with 60 ml of NaI solution (1.7 g cm-3) and 30 glass beads in a 250 ml centrifuge bottle and shaken for 16 hours on a reciprocal shaker. After shaking, samples were left undisturbed for 48 hours. The light fraction was then removed by suction into a vacuum filtration unit fitted with a 0.45 µm cellulose nitrate filter, washed with ~200 ml of distilled deionized water and dried at 60°C to a constant weight. Samples were re-suspended with another 60 ml of NaI solution, swirled vigorously and left to stand for 1 hour after which the LF was removed by suction. The remaining NaI in the HF was diluted with distilled deionized water and suctioned off by vacuum filtration. This was repeated until the solution was clear, after which the HF was dried at 60°C to a constant weight. We added 150 ml of deionized water to the HF and dispersed the soil for 16 hours with reciprocal shaking. After shaking, samples were dispersed over a 250-µm sieve nested on a 53-µm sieve and washed with deionized water until the wash water was clear. The fraction <53 µm was collected in a 1 l beaker and the <5 µm and 5−53 µm fractions were separated by gravity sedimentation according to Stokes’s Law. The <5 µm fraction was collected after each sedimentation in a 10 l bucket and calcium chloride (CaCl2) (1.5 g l-1) was added after all sedimentations were complete to flocculate the fine particles. All fractions were dried at 60 °C to a constant weight, which was recorded before the fractions were ground (< 250 µm) for further analysis.
Five fractions were isolated by this method:
- <1.7 g cm-3, light fraction
- < 2 mm to > 250 µm, coarse sand-HF
- < 250 µm to > 53 µm, fine sand-HF
- < 53 µm to > 5 µm, silt-HF
- < 5 µm, clay-HF.
To isolate relatively labile and stable SOM and understand where in the soil matrix these fractions are located.
To achieve several purposes we wish to isolate SOM fractions that can be:
- related to processes and mechanisms involved in stabilizing plant-derived C (and possibly nutrients such as N)
- used for modeling SOM dynamics and C storage
- quickly and efficiently recovered from soil i.e., minimize the workload
To be reproducible the method requires specialized training. It is best done by experienced personnel.
Diochon, A., Gillespie, A., Ellert, B., Janzen, H., Gregorich, E., 2016. Recovery and dynamics of decomposing plant residue in soil: an evaluation of three fractionation methods. European Journal of Soil Science 67, 196–205. doi: 10.1111/ejss.12316