Nitrogen deposition influences nitrogen isotope composition in soil and needles of Pinus massoniana forests along an urban-rural gradient in the Pearl River Delta of south China

Purpose Atmospheric nitrogen (N) deposition remains globally and regionally a significant N source in forest ecosystems, with intensive industrial activities. Stable N isotope ratio (δ¹⁵N) is a useful indicator widely adopted to assess environmental and ecological impacts of anthropogenic N inputs. On the basis of temporal changes in tree ring δ¹⁵N established recently, the present study investigated the influence of N deposition on δ¹⁵N in needles of Masson pine (Pinus massoniana L.) and forest soil along an urban-rural gradient in the Pearl River Delta of south China. Materials and methods Soil and needle samples were selected from South China Botanical Garden (SBG) in Guangzhou, Ding Hu Shan Natural Reserve (DHS) in Zhaoqing, and Nan Kun Shan Natural Reserve (NKS) in Huizhou. Five soil profiles at each site were sampled at three horizons: organic horizon (Oa), 0-10 and 10-20 cm depths, underneath the selected Masson pine trees. Soil samples were air-dried and sieved through 2 mm for analysis. Current-year and previous-year needles of Masson pine were collected from outer branches of the middle canopy at south, west, east, and north directions, and pooled according to needle age for each tree. Needles were rinsed, dried, ground, and stored in desiccator before analysis. Nitrogen isotope ratios were determined by Finnigan isotope mass spectrometer. Results and discussion Significant differences in soils (Oa and 0-10 cm depth layers) and needles along the urban-rural gradient were revealed with positively high δ¹⁵N values (+2.19 ± 1.43‰ in Oa, +6.67 ± 1.52‰ in 0-10 cm depth layer and about +1.0‰ in needles) at the rural site (NKS) and negatively low δ¹⁵N values (−5.51 ± 1.38‰ in Oa, −1.11 ± 1.36‰ in 0-10 cm depth layer and about −5.0‰ in needles) at the highest N deposition site (SBG). Needles exposed to high N deposition had significantly different δ¹⁵N values between age classes. The results suggested that atmospheric N deposition density contributed to the patterns of δ¹⁵N in soil and needles of Pinus massoniana along the urban-rural gradient. Conclusions The detectably decreasing trends of δ¹⁵N observed in the soils and pine needles from NKS to SBG implied that N deposition has influenced the forests N isotope composition. The patterns of soil- and needle-δ¹⁵N of Masson pine might be closely related to N deposition intensity. Values of δ¹⁵N in soils and needles were indicative of the gradient of urban-rural N deposition in the Pearl River Delta of south China.