Stable nitrogen isotope analysis of bulk tissue
The stable isotope ratios of nitrogen for all samples were measured using one of two EA-IRMS systems depending on the volume of each sample (see Text S2). Stable isotope ratios are expressed in δ notation in accordance with the international standard scale, on the basis of the following equation:
\(\delta^{15}N_{\text{Bulk}}\ =\ R_{\text{sample}}/R_{\text{standard}}\ \ 1\), (1)
where R sample is the 15N/14N ratio of the sample, andR standard is that of atmospheric nitrogen. Theδ 15N values were calibrated against those of alanine A (δ 15N = –2.89‰; CERKU-01), alanine B (δ 15N = 22.71‰; CERKU-02), and threonine (δ 15N = –2.88‰; CERKU-05) laboratory standards (Tayasu et al . 2011). The maximum analytical standard deviations (SD) of these standards were lower than 1.07‰ for alanine A (n = 40), 0.63‰ for alanine B (n = 54), and 0.15‰ for threonine (n = 8).