Free amino acid concentrations and nitrogen isotope signatures in Pinus massoniana (Lamb.) needles of different ages for indicating atmospheric nitrogen deposition

Free amino acid concentrations and nitrogen (N) isotopic composition in new current-year (new), mature current-year (middle-aged) and previous-year (old) Masson pine (Pinus massoniana Lamb.) needles were determined to indicate atmospheric N deposition in Guiyang (SW China). In different areas, free amino acids (especially arginine) concentrations in new and middle-aged needles were higher than in old needles, and the variation of free amino acids (especially arginine) concentrations in new and middle-aged needles was also greater than in old needles. This indicate that free amino acids in new and middle-aged needles may be more sensitive to N deposition compared to old needles. Moreover, concentrations of total free amino acids, arginine, histidine, γ-aminobutyric acid and alanine in middle-aged needles exhibited a strong relationship with N deposition (P < 0.05). Needle δ¹⁵N values showed a strong gradient from central Guiyang to the rural area, with more positive δ¹⁵N (especially in old needles) in the city center (0–5 km) and more negative δ¹⁵N (especially in old needles) in rural area (30–35 km). These suggest that N deposition in the urban center may be dominated by ¹⁵N-enriched NOx-N from traffic exhausts, while it is dominated by isotopically light atmospheric NHx-N from agriculture in rural area. Soil δ¹⁵N decreased slightly with distance from the city center, and the difference in δ¹⁵N values between the soil and needles (especially for old needles) increased significantly with the distance gradient, indicating that atmospheric N deposition may be an important N source for needles. This study provides novel evidence that free amino acids in needles and age-dependent needle δ¹⁵N values are useful indicators of atmospheric N deposition.