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

摘要

Nitrogen (N) concentrations and delta N-15 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 delta N-15 values in camphor and Masson pine leaves also decreased from central Guiyang to the rural area, with more positive leaf delta N-15 in the urban area and N-15-depleted leaf delta N-15 in the rural area. No significant differences were observed for soil N concentrations and soil delta N-15 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 delta N-15 values in urban areas were mainly influenced by N-15-enriched atmospheric NOx-N from traffic emissions, while those in rural areas were primarily affected by N-15-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) delta N-15 variation in the urban area (0 -12 km) was contrary to that of the camphor and Masson pine leaves, indicating that the delta N-15 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 delta N-15 values (P < 0.0001), and we thus applied a mass balance equation to estimate the delta N-15 value (about 7 parts per thousand) in the atmospheric N deposition in the urban area. This indicated that the greater delta N-15 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 delta N-15 in camphor and Masson pine leaves is a promising method to indicate N deposition. (C) 2017 Elsevier Ltd. All rights reserved.