Density fractions versus size separates: does physical fractionation isolate functional soil compartments?

[摘要] Physical fractionation is a widely used methodology to study soil organicmatter (SOM) dynamics, but concerns have been raised that the availablefractionation methods do not well describe functional SOM pools. In thisstudy we explore whether physical fractionation techniques isolate soilcompartments in a meaningful and functionally relevant way for the investigation oflitter-derived nitrogen dynamics at the decadal timescale. We do so byperforming aggregate density fractionation (ADF) and particle size-densityfractionation (PSDF) on mineral soil samples from two European beech forestsa decade after application of ¹⁵N labelled litter.

Both density and size-based fractionation methods suggested thatlitter-derived nitrogen became increasingly associated with the mineralphase as decomposition progressed, within aggregates and onto mineralsurfaces. However, scientists investigating specific aspects oflitter-derived nitrogen dynamics are pointed towards ADF when adsorption andaggregation processes are of interest, whereas PSDF is the superior tool toresearch the fate of particulate organic matter (POM).

Some methodological caveats were observed mainly for the PSDF procedure, themost important one being that fine fractions isolated after sonication cannot be linked to any defined decomposition pathway or protective mechanism.This also implies that historical assumptions about the "adsorbed" state ofcarbon associated with fine fractions need to be re-evaluated. Finally, thiswork demonstrates that establishing a comprehensive picture of whole soil OMdynamics requires a combination of both methodologies and we offer asuggestion for an efficient combination of the density and size-basedapproaches.

[学科分类] 地球化学与岩石