Nature of the occluded low-density fraction in soil organic matter studies: A critical review

Density separations show great promise in elucidating the progression of organic matter decomposition and mineral association in soils. We review the literature on these separations, with a focus on the low-density material released by sonication, the so-called occluded, aggregate-protected or mineral-associated low-density fraction (m-LF). This fraction accounts for up to half of the total C in surface soils. A commonly cited model explains this material as an intermediary (between mineral-free LF and high-density fractions) during the progressive decay of plant detritus accompanied by mineral association. However, the great variance in m-LF compositions (e.g. unusual aliphaticity, high C:N, variable mean residence time) shown in the literature implies a separate genesis for some of the organic matter in this fraction in some soils. Aspects of organic particle size and lipid composition of original plant sources deserve more attention. We propose a revision of the current model that allows for materials of widely varying lability in this pool. A combination of density separation with isotope tracers, detailed chemical characterization and other physical separation techniques are needed to improve models of soil organic matter dynamics linking the density fractions.