N in the food source was successively enlarged.
Individuals on low protein diet likely reserved
dietary protein for tissue maintenance rather than
catabolising it and excreting the nitrogen therein
(‘‘protein sparing’’; Gannes et al., 1997), which
results in the observed low fractionation in
N. On
the other hand Collembola feeding on fungal
hyphae with high food quality presumably catabolized
excess dietary protein and by this had a higher
fractionation rate. Similar to the present study
Ruess et al. (2004a) reported of a positive correlation
between the N concentration in the diet and
the d
15
N signature in nematophagous and fungivorous
Collembola. Overall, the results suggest that
the protein balance of a consumer significantly
affects the trophic level shift of
15
N, which is in
line with Pearson et al. (2003) and Gaye-Siessegger
et al. (2004b).