There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δ15N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha−1 yr−1, while the throughfall covered a larger range of 11.7–65.1 kg N ha−1 yr−1. Foliar N concentration, NO3− leaching to stream, and soil NO3− concentration were low and NO3− production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO3− leaching to stream, and negatively with pH values in soil and stream water. Foliar δ15N was between −6.6‰ and 0.7‰, and was negatively correlated with soil NO3− concentration and NO3− leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δ15N values. We proposed several potential mechanism that could contribute to the δ15N pattern, including (1) increased plant uptake of 15N-depleted soil NO3−, (2) foliage uptake of 15N-depleted NH4+, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.
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