The stable isotope analysis became a major tool in soil food weds studies. A shift in δ15N value of 3.4‰ is generally suggested as a mean enrichment in 15N per trophic level. However trophic fractionation is not uniform across different animal taxa and different tissues, and varies with a range of factors, including quality and isotopic composition of the diet. We performed a laboratory experiment aiming to compare the trophic fractionation of nitrogen isotopes (Δ15N) in different Collembola species fed with three fungi of different palatability. Collembolans were fed with single-species fungal diet during 75 days. There was a positive correlation between the whole body C:N ratio and reproduction rates of collembolans. We therefore used the C:N ratio as a proxy of nutritional status. In all species of collembolans, the trophic fractionation decreased strongly with increased whole body C:N ratio, and this factor explained most of the within-species variation in Δ15N. In addition, the Δ15N decreased with increased δ15N in the diet. The analysis of published data on the trophic fractionation of collembolans in laboratory experiments confirmed these conclusions. The mean trophic fractionation in most collembolan species studied in the laboratory was fairly similar. In contrast, field studies have documented a consistent difference in δ15N among different families of collembolans. In particular, in nearly all published cases Onychiuridae had δ15N higher than Isotomidae or Entomobryidae. A sharp contrast between laboratory-based estimates of the trophic fractionation (generally uniform in different species and families) and field data (which show a consistent difference in δ15N among different species and even families of collembolans) confirms that δ15N values of field-collected animals bear important information on the trophic position occupied by a particular species, and is not heavily affected by the species-specific differences in trophic fractionation. As the Δ15N in collembolans depends on their nutritional status, we suggest that the C:N values should be reported along with isotopic data to allow cross-studies comparison.
doi:10.1016/j.pedobi.2010.10.004

Saprotrophic fungi represent an important resource for a number of fungivorous and omnivorous soil animals,but little is known about the patterns of isotopic fractionation by soil fungi.We grew five common species of saprotrophic microfungi in laboratory cultures on simple artificial substrate based on carbohydrates derived either from C3orC4 plants.Fungal cultures were kept at15,20 or 25 C. Isotopic composition of carbon(13C/12C) and nitrogen(15N/14N) in bulk fungal tissue was determined after 11,21and32days.The fractionation of carbon and nitrogen stable isotopes was species-specific, but generally did not differ in C3-andC4-based growth media.The Zygomycete Mucor plumbeus did not differ in d13C from the carbon source used,though Ascomycetes(Alternaria alternata, Cladosporium cladosporioides, Trichoderma harzianum and Ulocladium botrytis) were depleted in heavy carbon relative to the carbon source by 0.5–0.9%. Three species were significantly depleted in 15N relative to the sodium nitrate that was used as a single source of nitrogen.In all species, d15N but not d13C tended to increase with the age of fungal colonies.The effect of temperature on d15N was weak and in consistent in different species.In contrast,all fungi except T. harzianum accumulated more 13C at25 1C than at 15 1C. The overall variation in the isotopic signatures of saprotrophic fungi growing in identical conditions reached 8%0 for d15N and 2.5%0 for d13C due to species-specific differences in the isotopic fractionation and the age of individual fungal colonies.This variation should be incorporated into the interpretation of the isotopic composition of fungivorous soil animals.
doi:10.1016/j.pedobi.2009.11.001