ALBERT

Description: Physical fractionation is a widely used methodology to study soil organic matter (SOM) dynamics, but concerns have been raised that the available fractionation methods do not well describe functional SOM pools. We also examine the question whether physical fractionation techniques isolate ecologically meaningful, functionally relevant soil compartments. In this study we explore whether the kind of information that aggregate density fractionation (ADF) and particle size-density fractionation (PSDF) yield on soil OM dynamics is method-specific, similar, or complimentary. We do so by following the incorporation of a 15N label into mineral soils of two European beech forests a decade after its application as 15N labelled litter. Both density and size-based fractionation methods suggested that OM became increasingly associated with the mineral phase as decomposition progressed, within aggregates and onto mineral surfaces. Our results suggest that physical fractionation methods do isolate ecologically relevant functional soil subunits. However, scientists investigating specific aspects of OM dynamics are pointed towards ADF when adsorption and aggregation processes are of interest, whereas PSDF is the superior tool to research the fate of particulate organic matter (POM). Some methodological caveats were observed mainly for the PSDF procedure, the most important one being that fine fractions isolated after sonication can not be linked to any defined decomposition pathway or stabilisation process. This also implies that historical assumptions about the "adsorbed" state of carbon associated with fine fractions need to be re-evaluated. Finally, this work demonstrates that establishing a comprehensive picture of whole soil OM dynamics requires a combination of both methodologies and we offer a suggestion for an efficient combination of the density and size-based approaches.