Compound-specific δ¹⁵N composition of free amino acids in moss as indicators of atmospheric nitrogen sources

Haplocladium microphyllum moss samples were collected in Nanchang, China. Free amino acid (FAA) concentrations and N isotope compositions (δ¹⁵N_FAA) in the samples were determined and compared with the bulk N concentrations and δ¹⁵N_bulk values. The aim was to determine whether δ¹⁵N_FAA values in moss (which are very variable) indicate the sources of atmospheric N. The δ¹⁵N_FAA values among individual FAA varied widely (from -19.3‰ to +16.1‰), possibly because of the different sources of N and isotope fractionation in amino acids metabolic pathways. Total ¹⁵N-enrichment for the individual FAAs was equal to total ¹⁵N-depletion relative to δ¹⁵N_bulk. The concentration-weighted mean δ¹⁵N value for total FAAs (TFAA) (δ¹⁵N_TFAA) was -3.1‰ ± 3.2‰, which was similar to δ¹⁵N_bulk (-4.0‰ ± 2.9‰). We concluded that a N isotope balance occurred during amino acid metabolism and that little isotope disparity occurred between the concentration-weighted TFAA and bulk N. We concluded that δ¹⁵N_TFAA ≈ δ¹⁵N_bulk ≈ δ¹⁵N_source. The mean δ¹⁵N_alanine (-4.1‰), δ¹⁵N_glutamate (-4.2‰), and δ¹⁵N_lysine (-4.0‰) were similar to the mean δ¹⁵N_bulk, which we attributed to little isotope fractionation occurring during their in situ the metabolic pathways. This suggests that δ¹⁵N_alanine, δ¹⁵N_glutamate, and δ¹⁵N_lysine in moss can be used to indicate the sources of atmospheric N deposition.