Nature of the “occluded” low-density fraction in soil organic matter studies: A critical review
Version of Record online: 28 JAN 2009
© 2009 Japanese Society of Soil Science and Plant Nutrition
Soil Science & Plant Nutrition
Volume 55, Issue 1, pages 13–25, February 2009
How to Cite
WAGAI, R., MAYER, L. M. and KITAYAMA, K. (2009), Nature of the “occluded” low-density fraction in soil organic matter studies: A critical review. Soil Science & Plant Nutrition, 55: 13–25. doi: 10.1111/j.1747-0765.2008.00356.x
- Issue online: 28 JAN 2009
- Version of Record online: 28 JAN 2009
- Received 31 March 2008.; Accepted for publication 21 November 2008.
- aggregate protection;
- conceptual model;
- mineral coating;
- organo-mineral association;
- soil carbon stabilization
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.