Nitrogen deposition and forest nitrogen cycling along an urban-rural transect in southern China

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 delta 15N 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 delta 15N was between -6.6 parts per thousand and 0.7 parts per thousand, 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 delta 15N values. We proposed several potential mechanism that could contribute to the delta 15N 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.