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

Publication date: June 2017
Source:Atmospheric Environment, Volume 159
Author(s): Yu Xu, Huayun Xiao, Linglu Qu
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.



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