Enhancing the understanding of earthworm feeding behaviour via the use of fatty acid δ13C 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 (delta C-13 = -31.3 parts per thousand; delta N-15 = 10.7 parts per thousand) and C-4 (delta C-13 = -12.6 parts per thousand; delta N-15 = 7.5 parts per thousand) synthetic dung pats applied to a temperate grassland (delta C-13 = -30.3 parts per thousand; delta N-15 = 5.7 parts per thousand) in an experiment carried out for 372 days. Bulk delta C-13 values of earthworms collected from beneath either C-3 or C-4 dung after 28, 56, 112 and 372 days demonstrated that W L. rubellus beneath C-4 dung were significantly C-13-enriched after 56 days (delta C-13 = -23.8 parts per thousand) and 112 days (delta C-13 = -22.4 parts per thousand) compared with those from C-3 dung treatments (56 days, delta C-13 = -26.5 parts per thousand; 112 days, delta C-13 = -27.0 parts per thousand), and (ii) A. chlorotica were 2.1 parts per thousand C-13-enriched (delta C-13 = -24.2 parts per thousand) relative to those from C-3 dung (delta C-13 = -26.3 parts per thousand) treatments after 372 days. Bulk delta N-15 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 delta(15) N = 3.3 parts per thousand) had higher delta N-15 values than the epigeic species (total mean delta N-15 = 5.4 parts per thousand). Although the two species exhibited similar fatty acid profiles, individual fatty acid delta C-13 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 (iC(17:0), aC(17:0)) from C-13-labelled dung in situ, the routing of dung C into de novo synthesised compounds (C-20:4 omega 6, C-20:5 omega 3), and the assimilation of essential fatty acids (C-18:1 omega 9, C-18:1 omega 7, C-18:2 omega 6, C-18:3 omega 3) derived from dung, were determined. Copyright (C) 2008 John Wiley & Sons, Ltd.