Reservations about preservations: storage methods affect δ13C signatures differently even in closely related soil fauna

Abstract

Summary1. Studies of stable isotope signatures can reveal and quantify trophic carbon transfer between organisms. However, preservation of the samples before analysis cannot always be avoided. Some preservation agents are known to alter tissue δ13C values considerably, but we do not yet understand how variation in such preservation artefacts may be determined by variation in body traits of different invertebrate species and life stage.2. Here, we tested the effect of four different preservation methods on 13C signatures of two morphologically and ecologically distinct springtails, Folsomia candida and Orchesella cincta. These springtails were fed on the fungus Cladosporium cladosporioides grown on either a C3 or a C4 carbon source, resulting in springtails with two contrasting initial δ13C values. Subsequently, these springtails were preserved for 46 days. In addition, a juvenile–adult comparison was made for F. candida.3. Freeze‐drying and subsequent dry storage did not affect 13C signatures of either species; nor did killing springtails with liquid nitrogen and storing them at −80 °C. Preservation in 70% ethanol slightly depleted δ13C values of adult F. candida but not of O. cincta. In contrast, storage in saturated salt solution depleted both species considerably. Life stage affected preservation success significantly; storage in 70% ethanol depleted adult F. candida but not its juveniles. Initial δ13C values of the springtails did not interact with preservation artefacts, suggesting that the shifts in δ13C values are caused by effects of the preservative on the animal tissue rather than by its remainders.4. We recommend freeze‐drying as a preservation method. However, our results suggest that interspecific differences (e.g. in body size and cuticle thickness) as well as intraspecific difference (e.g. life stage‐dependent changes in the proportion of fat reserves) are important determinants of preservation effects on 13C signatures. This makes interpreting stable isotope data obtained from preserved springtails relatively difficult, especially when natural 13C abundances are used to study trophic interactions or interspecific functional differences. We predict that such complications also apply to other invertebrate taxa and types.