From Litter to Cast: Earthworm-Mediated Lignin Dynamics across Contrasting Soil Types

Lignin dynamics are critical for soil organic matter (SOM) stability and carbon sequestration, yet how earthworm species belonging and soil properties jointly regulate lignin enrichment, transformation, and partitioning between bulk casts and mineral-associated organic matter (MAOM) remains unclear. Here, we employed complementary analytical approaches—including CuO oxidation, TMAH thermochemolysis, and ¹³C analysis of CuO oxidation-derived lignin monomers—to investigate three earthworm species (litter dweller Lumbricus castaneus [LC], burrower Lumbricus terrestris [LT], deep bioturbator Allolobophora icterica [AI]) in two contrasting soils (calcareous Luvisol, acidic Cambisol). Lignin concentrations in bulk casts and MAOM were 2.1–3.0 times higher than control soils, with distinct species-specific signatures (LC>LT>AI) tied to feeding and burrowing traits. Luvisol favored methoxylated lignin preservation (high S/V ratio) via calcium-mediated protection, while Cambisol promoted oxidative transformation (elevated acid-to-aldehyde ratios). ¹³C analysis of MAOM lignin confirmed litter-derived contributions to this fraction. Our findings demonstrate that earthworm species belonging and soil chemistry synergistically drive lignin dynamics, challenging the notion of uniform earthworm effects on SOM composition. These insights highlight the need to incorporate species-specific traits and soil properties into SOM dynamics and carbon cycling models, informing sustainable soil management.