Atmospheric nitrogen (N) deposition remains globally and regionally a significant N source in forest ecosystems, with intensive industrial activities. Stable N isotope composition (δ15N) in tree-rings of Masson pine (Pinus Massoniana L.) was investigated to assess the historical changes in plant availability of N along an urban–rural gradient in the Pearl River Delta of south China based on the detective variations of δ15N in the needles and soils of the Masson pine forests.

Patterns of δ15N in tree rings at the three sites revealed a common trend decreasing from the pith to the recently formed xylem though fluctuations were observed during the certain periods. Tree ring δ15N showed the most significant decrease at urban area from the 1960s to the late-1990s. Minor decrease of δ15N in pine trees at rural area was observed with small fluctuations over time. Moreover, a slight rally of δ15N since the late 1980s was observed at urban site. On average, the dendroisotopic series showed much lower δ15N values at urban site than the other sites.

The detectably decreasing trends of δ15N observed in the tree-rings at the selected sites implied that N deposition has influenced the plant N availability. The patterns of tree-ring δ15N of Masson pine might be closely related to N deposition intensity.