Nitrogen concentrations and nitrogen isotopic compositions in leaves of Cinnamomum Camphora and Pinus massoniana (Lamb.) for indicating atmospheric nitrogen deposition in Guiyang (SW China)

Abstract

Nitrogen (N) concentrations and δ15N signatures in soil and camphor (Cinnamomum Camphora) and Masson pine (Pinus massoniana Lamb.) leaves collected along an urban-rural gradient in Guiyang (SW China) were investigated systematically. N concentrations in camphor (1.01–2.37%) and Masson pine (0.99–2.42%) leaves showed a significant decrease from central Guiyang (0–6 km) to suburban areas (18–24 km), while slightly increased leaf N concentrations reemerged at areas more than 24 km from the city center. The δ15N values in camphor and Masson pine leaves also decreased from central Guiyang to the rural area, with more positive leaf δ15N in the urban area and 15N-depleted leaf δ15N in the rural area. No significant differences were observed for soil N concentrations and soil δ15N in these areas, which suggested that the decrease in leaf N concentrations was due to decreased atmospheric N deposition along the urban-rural gradient and that there were two isotopically different atmospheric N sources in Guiyang city: foliar δ15N values in urban areas were mainly influenced by 15N-enriched atmospheric NOx-N from traffic emissions, while those in rural areas were primarily affected by 15N-depleted atmospheric NHx-N from agricultural activities. However, the pattern of moss (collected ten years prior, with lower traffic density and wastewater treatment rate in the urban area) δ15N variation in the urban area (0–12 km) was contrary to that of the camphor and Masson pine leaves, indicating that the δ15N values in previously collected urban mosses were mainly controlled by isotopically light NHx-N from untreated wastes and sewage, but were much less affected by traffic. For the trees in the urban area, N concentrations in camphor and Masson pine leaves varied in parallel with their δ15N values (P < 0.0001), and we thus applied a mass balance equation to estimate the δ15N value (about 7‰) in the atmospheric N deposition in the urban area. This indicated that the greater δ15N in urban camphor and Masson pine leaves reflected a higher contribution of NOx-N to N deposition. This study shows that the analysis of N and δ15N in camphor and Masson pine leaves is a promising method to indicate N deposition.