Compound-specific δ(15)N 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 (δ(15)N(FAA)) in the samples were determined and compared with the bulk N concentrations and δ(15)N(bulk) values. The aim was to determine whether δ(15)N(FAA) values in moss (which are very variable) indicate the sources of atmospheric N. The δ(15)N(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 (15)N-enrichment for the individual FAAs was equal to total (15)N-depletion relative to δ(15)N(bulk). The concentration-weighted mean δ(15)N value for total FAAs (TFAA) (δ(15)N(TFAA)) was −3.1‰ ± 3.2‰, which was similar to δ(15)N(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 δ(15)N(TFAA) ≈ δ(15)N(bulk) ≈ δ(15)N(source). The mean δ(15)N(alanine) (−4.1‰), δ(15)N(glutamate) (−4.2‰), and δ(15)N(lysine) (−4.0‰) were similar to the mean δ(15)N(bulk), which we attributed to little isotope fractionation occurring during their in situ the metabolic pathways. This suggests that δ(15)N(alanine), δ(15)N(glutamate), and δ(15)N(lysine) in moss can be used to indicate the sources of atmospheric N deposition.