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Earthworms are integral parts of many ecosystems and may play a decisive role in determining whether soils function as carbon (C) sink or source. However, information on how earthworms affect the composition and stability of soil organic matter (SOM) is scarce. Particularly their effect on organic matter deriving from leaves and roots with distinct composition and, thus, susceptibility to decomposition and stabilization remains unclear. Here, we combine cutin- and suberin-derived lipids as specific markers for leaf- and root-derived SOM with their 13C composition and physical fractionations of soil. We show that earthworms overprint the protective role of organo-mineral associations and aggregates to favor the accumulation of root- relative to leaf-derived SOM. This gradual accumulation contributes to the often-observed dominance of root-derived organic matter in soil and emphasizes the need to consider molecular level effects of earthworms on SOM dynamics.

Earthworms are integral parts of many ecosystems and may play a decisive role in determining whether soils function as carbon (C) sink or source. However, information on how earthworms affect the composition and stability of soil organic matter (SOM) is scarce. Particularly their effect on organic matter deriving from leaves and roots with distinct composition and, thus, susceptibility to decomposition and stabilization remains unclear. Here, we combine cutin- and suberin-derived lipids as specific markers for leaf- and root-derived SOM with their 13C composition and physical fractionations of soil. We show that earthworms overprint the protective role of organo-mineral associations and aggregates to favor the accumulation of root- relative to leaf-derived SOM. This gradual accumulation contributes to the often-observed dominance of root-derived organic matter in soil and emphasizes the need to consider molecular level effects of earthworms on SOM dynamics.

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.