Labeling earthworms uniformly with 13C and 15N: implications for monitoring nutrient fluxes

Stable isotopes hold promise for improving our ability to quantify energy and N released from earthworm populations through metabolic processes and mortality. However, the isotopic labels 13C and 15N must be incorporated uniformly into the structural and labile tissues of earthworms to trace C and N fluxes accurately. We examined the distribution of 13C and 15N in the tissue and mucus of newly hatched, juvenile and adult Aporrectodea tuberculata (Eisen) fed double-labeled (13C and 15N) wheat for 4, 8, 12 and 16 weeks. After 4 weeks, earthworm tissue and mucus contained up to 1.273 at.% 13C and 0.389 at.% 15N. The 13C and 15N enrichment in hatchlings increased significantly (P<0.05) between 4 and 16 weeks, but did not change in juvenile and adult earthworms. The 13C enrichment of earthworm tissue and mucus was not uniform because some of the 13C in earthworm tissue was respired, but 15N was uniformly distributed in earthworm tissue and mucus. We show that earthworms can be uniformly labeled with 15N, but not 13C, as soon as 4 weeks after earthworms begin feeding on double-labeled litter. Our findings indicate N turnover and excretion rates can be calculated accurately from 15N tracer studies, and may improve estimates of N flux from earthworm populations.