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

Abstract

Molecular and compound specific isotope compositions of apolar lipids were characterized in soil mesocosms incubated for one year with or without 13 C-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 (Clab). Casts displayed higher Clab values than surrounding soil while soil without earthworm exhibited intermediate Clab. 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 Clab 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 13 C-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.

Earthworms as catalysts in the formation and stabilization of soil microbial necromass

“…While the central role of earthworms in the formation of microbial SOM is supported by recent experimental evidence (increases in microbial‐derived amino sugars between 37% and 145% have been reported; Angst, Mueller, Prater, et al, 2019; Mora et al, 2003; Nguyen Tu et al, 2020; Vidal et al, 2019), the relevance of our concept to bulk SOC stability and stocks in various environments, for example, different soil groups, land uses, or covers (see also Figure 1), and in the context of a changing climate remains unclear (Phillips et al, 2019; Singh et al, 2019). The potential of earthworms to increase N 2 O emissions (Drake & Horn, 2007; Lubbers et al, 2013) and the fact that invasive species may initially reduce overall soil carbon stocks (forest floor + mineral soil; Bohlen et al, 2004, but see also Ferlian et al, 2020) have to further be reconciled with the favorable effect of earthworm activity on microbial necromass formation reported here.…”