Influence of earthworms on apolar lipid features in soils after 1 year of incubation

Molecular and compound specific isotope compositions of apolar lipids were characterized in soil mesocosms incubated for 1 year with or without ¹³C-labelled plant residues and earthworms, in order to investigate, at the molecular scale, the effect of earthworms on the fate of organic matter (OM) in soils. Molecular and isotope composition of long chain alkanes in casts confirmed that earthworms preferentially ingest soil fractions rich in plant debris. Apolar lipid specific isotope composition allowed calculation of the proportion of carbon derived from the labelled residues (C_lab). Casts displayed higher C_lab values than surrounding soil while soil without earthworm exhibited intermediate C_lab. The odd-over-even predominance (OEP) of alkanes suggested they are probably less degraded in casts than in the surrounding soil. Taken together, OEP and C_lab values suggested that besides high incorporation of plant residues, earthworms may also favor the preservation of plant apolar lipids in their casts. Additionally, chain length and isotope pattern of alkanes further suggested root lipids were probably less degraded than shoot lipids. High ¹³C-incorporation level for the bacterial biomarker hopene provided evidence for intense recycling of plant OM and suggested further contribution of bacterial necromass to soil OM.