Enhancing the understanding of earthworm feeding behaviour via the use of fatty acid 13C 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 C3 (13C = -31.3; 15N = 10.7) and C4 (13C = -12.6; 15N = 7.5) synthetic dung pats applied to a temperate grassland (13C = -30.3; 15N = 5.7) in an experiment carried out for 372 days. Bulk 13C values of earthworms collected from beneath either C3 or C4 dung after 28, 56, 112 and 372 days demonstrated that (i) L. rubellus beneath C4 dung were significantly 13C-enriched after 56 days (13C = -23.8) and 112 days (13C = -22.4) compared with those from C3 dung treatments (56 days, 13C = -26.5; 112 days, 13C = -27.0), and (ii) A. chlorotica were 2.1 13C-enriched (13C = -24.2) relative to those from C3 dung (13C = -26.3) treatments after 372 days. Bulk 15N values did not suggest significant uptake of dung N by either species beneath C3 or C4 dung, but showed that the endogeic species (total mean 15N = 3.3) had higher 15N values than the epigeic species (total mean 15N = 5.4). Although the two species exhibited similar fatty acid profiles, individual fatty acid 13C 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 (iC17:0, aC17:0) from 13C-labelled dung in situ, the routing of dung C into de novo synthesised compounds (, ), and the assimilation of essential fatty acids (, , , ) derived from dung, were determined