Enhancing the understanding of earthworm feeding behaviour via the use of fatty acid δ 13 C values determined by gas chromatography‐combustion‐isotope ratio mass spectrometry

Litter‐dwelling (epigeic) Lumbricus rubellus and soil‐dwelling (endogeic) Allolobophora chlorotica earthworms were observed aggregating under C 3 ( δ 13 C = −31.3‰; δ 15 N = 10.7‰) and C 4 ( δ 13 C = −12.6‰; δ 15 N = 7.5‰) synthetic dung pats applied to a temperate grassland ( δ 13 C = −30.3‰; δ 15 N = 5.7‰) in an experiment carried out for 372 days. Bulk δ 13 C values of earthworms collected from beneath either C 3 or C 4 dung after 28, 56, 112 and 372 days demonstrated that (i) L. rubellus beneath C 4 dung were significantly 13 C‐enriched after 56 days ( δ 13 C = −23.8‰) and 112 days ( δ 13 C = −22.4‰) compared with those from C 3 dung treatments (56 days, δ 13 C = −26.5‰; 112 days, δ 13 C = −27.0‰), and (ii) A. chlorotica were 2.1‰ 13 C‐enriched ( δ 13 C = −24.2‰) relative to those from C 3 dung ( δ 13 C = −26.3‰) treatments after 372 days. Bulk δ 15 N values did not suggest significant uptake of dung N by either species beneath C 3 or C 4 dung, but showed that the endogeic species (total mean δ 15 N = 3.3‰) had higher δ 15 N values than the epigeic species (total mean δ 15 N = 5.4‰). Although the two species exhibited similar fatty acid profiles, individual fatty acid δ 13 C values revealed extensive routing of dietary C into body tissue of L. rubellus , but minor incorporation into A. chlorotica . In particular, the direct incorporation of microbial biomarker fatty acids ( i C 17:0 , a C 17:0 ) from 13 C‐labelled dung in situ , the routing of dung C into de novo synthesised compounds ( ${\rm C}_{20:4_\omega 6}$ , ${\rm C}_{20:5_\omega 3}$ ), and the assimilation of essential fatty acids ( ${\rm C}_{18:1_\omega 9}$ , ${\rm C}_{18:1_\omega 7}$ , ${\rm C}_{18:2_\omega 6}$ , ${\rm C}_{18:3_\omega 3}$ ) derived from dung, were determined. Copyright © 2008 John Wiley & Sons, Ltd.