Abundance of lipids in differently sized aggregates depends on their chemical composition

Abstract

Evidence for a vital role of soil mineral matrix interactions in lipid preservation is steadily increasing. However, it remains unclear whether solvent-extractable (‘free’) or hydrolyzable (‘bound’) lipids, including molecular proxies, e.g., for cutin and suberin, are similarly affected by different stabilization mechanisms in soil (i.e., aggregation or organo-mineral association). To provide insights into the effect of these stabilization mechanisms on lipid composition and preservation, we investigated free and bound lipids in particulate and mineral soil fractions, deriving from sand- and silt-/clay-sized aggregates from a forest subsoil. While free lipids accumulated in sand-sized aggregates, the more complex bound lipids accumulated in silt- and clay-sized aggregates, particularly in the respective mineral fractions < 6.3 µm (fine silt and clay). The presence of both, cutin and suberin markers indicated input of leaf- and root-derived organic matter to the subsoil. Yet, our cutin marker (9,10,ω-trihydroxyoctadecanoic acid) was not extracted from the mineral aggregate compartments < 6.3 µm, perhaps due to its chemical structure (i.e., cross-linking via several hydroxy groups, and thus higher ‘stability’, in macromolecular structures). Combined, these results suggest that the chemical composition of lipids (and likely also that of other soil organic matter compounds) governs interaction with their environment, such as accumulation in aggregates or association with mineral soil compartments, and thus indirectly influences their persistence in soil.