Isotope analysis and %N
Freeze-dried samples (powdered total organic matter) were weighed in tin
capsules, and δ15N values and %N were measured using
an elemental analyser (Carlo-Erba) coupled to isotope ratio mass
spectrometer (Isochrom, Elementar) run in continuous flow. All sample
batches included standards (glycine, +0.66‰; cysteine, +7.78‰;
previously calibrated against IAEA standards glutamic acid USGS-40 and
caffeine IAEA-600) each ten samples.