Free amino acid δ¹⁵N values and concentrations of current-year new (new), current-year mature (middle-age) and previous-year (old) Pinus massoniana (Lamb.) needles were determined for five sites with different distances from a highway in a forest in Guiyang (SW China). Needle free amino acid concentrations decreased with increasing distance from the highway, and only the free amino acid concentrations (total free amino acid, arginine, γ-aminobutyric acid, valine, alanine and proline) in the middle-aged needles demonstrated a strong correlation with distance from the highway, indicating that free amino acid concentrations in middle-aged needles may be a more suitable indicator of nitrogen (N) deposition compared to new and old needles. Needle free amino acid δ¹⁵N values were more positive near the highway compared to the more distant sites and increased with increasing needle age, indicating that N deposition in this site may be dominated by isotopically heavy NO_x-N from traffic emissions. In sites beyond 400 m from the highway, the δ¹⁵N values of total free amino acids, histidine, glutamine, proline, alanine, aspartate, isoleucine, lysine, arginine and serine in each age of needle were noticeably negative compared to their respective δ¹⁵N values near the highway. This suggested that needle free amino acid δ¹⁵N values from these sites were more affected by ¹⁵N-depleted atmospheric NH_x-N from soil emissions. This result was further supported by the similarity in the negative moss δ¹⁵N values at these sites to the δ¹⁵N values of soil-derived NH_x-N. Needle free amino acid δ¹⁵N values therefore have the potential to provide information about atmospheric N sources. We conclude that needle free amino acid concentrations are sensitive indicators of N deposition and that the age-related free amino acid δ¹⁵N values in needles can efficiently reflect atmospheric N sources. This would probably promote the application of the combined plant tissue amino acid concentration and δ¹⁵N analyses in N deposition bio-monitoring.