The role of earthworms in the transformation of labile plant litter into mineral-associated organic matter

Abstract

Although it has been recognized that the formation of organo-mineral complexes may be fostered by soil fauna, their formation pathways are far from understood. The objective of this study was to investigate the processes involved in earthworm-mediated mineral-associated organic matter (MAOM) formation. To this end, we used microcosms to produce casts with three different temperate earthworm species (epigeic Lumbricus castaneus [LC], epi-anecic Lumbricus terrestris [LT], endogeic Aporrectodea icterica [AI]) fed with ¹³C-enriched Miscanthus litter in two contrasting soil types (Luvisol, Cambisol). To investigate MAOM formation processes, we used ¹³C isotope tracing to track litter-derived carbon origin and neutral sugar biomarkers to resolve microbial transformation pathways.

Our results indicated that cast MAOM-C concentrations increased compared to the control treatment without earthworms, with LC casts in Luvisol exhibiting the highest MAOM-C concentration (24.1 mg g⁻¹). MAOM-C contribution to casts decreased in the order LC > LT > AI. Sugar biomarkers revealed species- and soil-dependent stabilization pathways: MAOM of LC casts in Luvisol contained high proportions of plant-derived arabinose, while MAOM of AI casts in Cambisol were enriched in microbial-derived hexoses (mannose) and deoxy-hexoses (rhamnose, fucose). We conclude that earthworms depending on species and soil type accelerated MAOM formation via synergistic pathways: (1) direct stabilization of plant sugars and (2) microbial transformation of labile litter into necromass during cast formation. We suggest that earthworm-driven MAOM formation depends on species-specific traits (e.g., LC's capacity for litter fragmentation) and soil properties (e.g., clay and CaCO₃ content).