Haplocladium microphyllum moss samples were collected in Nanchang, China. Free amino acid (FAA) concentrations and N isotope compositions (δ15NFAA) in the samples were determined and compared with the bulk N concentrations and δ15Nbulk values. The aim was to determine whether δ15NFAA values in moss (which are very variable) indicate the sources of atmospheric N. The δ15NFAA 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 δ15Nbulk. The concentration-weighted mean δ15N value for total FAAs (TFAA) (δ15NTFAA) was -3.1‰ ± 3.2‰, which was similar to δ15Nbulk (-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 δ15NTFAA ≈ δ15Nbulk ≈ δ15Nsource. The mean δ15Nalanine (-4.1‰), δ15Nglutamate (-4.2‰), and δ15Nlysine (-4.0‰) were similar to the mean δ15Nbulk, which we attributed to little isotope fractionation occurring during their in situ the metabolic pathways. This suggests that δ15Nalanine, δ15Nglutamate, and δ15Nlysine in moss can be used to indicate the sources of atmospheric N deposition.