Concentrations and nitrogen isotope compositions of free amino acids in Pinus massoniana (Lamb.) needles of different ages as indicators of atmospheric nitrogen pollution

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 NOx-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 NHx-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 NHx-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.