Technical note: The recovery rate of free particulate organic matter is strongly reduced by conventional density fractionation of soil samples
- Chair of Soil Science, Dept. of Ecology, Technische Universität Berlin, 10587 Berlin, Germany
Abstract. Ultrasonication combined with density fractionation (USD) is a method widely used to quantify 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 sample and dense medium. It could be shown that pouring the dense solution to the mineral matrices 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, the results confirmed the low extraction rate of the fill-up approach. Moreover, it was possible to show 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 among the tested methods to achieve complete and selective extraction of fPOM from natural soil samples.
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