Production of rabbit dung triple-labelled with C, N and S stable isotopes

Animal dung is an important resource in natural, pastoral and agro-ecosystems. Labelling dung with stable isotopes offers a powerful technique to trace faeces-derived carbon and nutrients in soils and organisms, but the production of isotopically labelled animal excrements needs to be practicable and cost-effective. Here we present a simple method for producing solid rabbit faeces triple-labelled with 13C, 15N and 34S stable isotopes. The steps involved are: (1) fertilising cereal seedlings with isotopically enriched fertilizers (13C–15N urea and 34S sodium sulphate); (2) feeding these seedlings daily to a rabbit as supplementary forage for 6 days; and (3) collecting the accruing faeces and measuring the isotopic enrichment in bulk dung and the undigested fraction. The rabbit dung was clearly enriched in 13C, 15N and 34S compared to unlabelled start dung. The enrichments increased linearly with time and peaked on the last day of the labelling diet, but most were still detectable two days later, especially 15N. The undigested fraction (Neutral Detergent Fibre) had lower enrichments than the bulk material; this was especially marked for 34S. Data from a follow-up experiment, in which soil microcosms were fertilised with the dung, illustrated that the 15N enrichments were sufficient to track N incorporation into grass. Our method is simple, rapid and suitable to the small-scale production of labelled faeces. We present tracer recovery estimates for the sequence total tracer used ⇒ wheat biomass ⇒ animal dung and discuss ways of increasing the proportions recovered and of obtaining higher enrichments if required.