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.