Technical note: The recovery rate of free particulate organic matter from soil samples is strongly affected by the method of density fractionation

Ultrasonication combined with density fractionation (USD) is a method widely used to separate soil organic matter pools. A selective fractionation of free particulate organic matter (fPOM) is crucial to avoid co-extraction of retained fPOM along with occluded particulate organic matter (oPOM). In the present work, artificial fPOM was extracted from two mineral matrices, sandy and loamy, after applying different approaches for merging a sample and dense medium. It is shown that pouring the dense solution to the mineral matrices without mixing leads to low recovery, whereas trickling the sample into the solution, rotating after fill-up or applying a minimal and defined amount of ultrasound to swirl up the sample causes nearly full recovery of the artificial fPOM. Applied to natural soils, our results confirmed the low extraction rate of the unmixed approach. It was also further shown that the rotational approach results in only a slightly increased extraction rate, whereas the ultrasound approach leads to a release of oPOM into the fPOM fraction due to disruption of soil macro-aggregates. The trickle approach appears to be the most appropriate way from the tested methods to achieve complete and selective extraction of fPOM from natural soil samples.