Nitrogen deposition promotes ecosystem carbon accumulation by reducing soil carbon emission in a subtropical forest

Tropical and subtropical forests are experiencing high levels of atmospheric nitrogen (N) deposition, but the responses of such forests ecosystems to N deposition remain poorly understood. We conducted an 8-year field experiment examining the effect of experimental N deposition on plant growth, soil carbon dioxide efflux, and net ecosystem production (NEP) in a subtropical Chinese fir forest. The quantities of N added were 0 (control), 60, 120, and 240 kg ha(-1) year(-1). NEP was lowest under ambient conditions and highest with 240 kg of N ha(-1) year(-1) treatment. The net increase in ecosystem carbon (C) storage ranged from 9.2 to 16.4 kg C per kg N added in comparison with control. In addition, N deposition treatments significantly decreased heterotrophic respiration (by 0.69-1.85 t C ha(-1) year(-1)) and did not affect plant biomass. The nitrogen concentrations were higher in needles than that in fine roots. Our findings suggest that the young Chinese fir forest is carbon source and N deposition would sequester additional atmospheric CO2 at high levels N input, mainly due to reduced soil CO2 emission rather than increased plant growth, and the amount of sequestered C depended on the rate of N deposition.