Changes to vertebrate tissue stable isotope (δ ¹⁵N) composition during decomposition

Abstract

During carcass decomposition, tissues undergo biochemical changes: Cells autolyze, enteric microbes ferment cellular products, and tissues degrade. Ultimately, decomposition fluids are released as an ephemeral nitrogen (N) and carbon source to the surrounding environment. However, decomposition fluids are δ ¹⁵N-enriched relative to body tissues, leading to a disconnect between starting tissue composition and ending fluid composition. It remains largely unknown when or if tissues exhibit δ ¹⁵N enrichment postmortem despite the importance of tissue stable isotopes to ecologists. To test our hypothesis that tissues would become progressively δ ¹⁵N-enriched during decay, soft tissues and bone were collected from beaver carcasses at five time points. All soft tissues, including muscle, were significantly δ ¹⁵N-enriched compared to fresh tissues, but were not as enriched as decomposition fluids. Tissue breakdown is initially dominated by anaerobic autolysis and later by microbe and insect infiltration, and partly explains decay fluid isotopic enrichment. We speculate that after rupture, preferential volatilization of δ ¹⁵N-depleted compounds (especially ammonia) contributes to further enrichment. These results constrain the timing, rate, and potential mechanisms driving carcass isotopic enrichment during decay, and suggest that found carcasses (e.g., road kill) should be used with caution for inferring trophic ecology as decay can result in significant postmortem δ ¹⁵N enrichment.