Atmospheric nitrogen (N) deposition greatly affects ecosystem processes and properties. However, few studies have simultaneously examined the responses of both the above- and belowground communities to N deposition. Here, we investigated the effects of 8 years of simulated N deposition on plant growth and understory plant diversity, soil carbon dioxide efflux, net ecosystem production (NEP), soil microbial communities in a subtropical forest. The quantities of experimental N added (g of N m-2 yr-1) and treatment codes were 0 (N0, control), 6 (N1), 12 (N2), and 24 (N3). Phospholipid fatty acids (PLFAs) analysis was used to characterize the soil microbial community while plant diversity and coverage were determined in the permanent field plots.
Results/Conclusions
The results indicated that NEP was lowest under ambient conditions and highest with 240 kg of N ha-1yr-1 treatment. The net increase in ecosystem carbon (C) storage ranged from 9.2–16.4 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 yr-1) and did not affect plant biomass. The nitrogen concentrations were higher in needles than that in fine roots. Microbial abundance was reduced by the N3 treatment, and plant species richness and coverage were reduced by both N2 and N3 treatments. Declines in plant species richness were associated with decreased abundance of arbuscular mycorrhizal fungi, increased bacterial stress index, and reduced soil pH. The plasticity of soil microbial community would be more related to the different responses among treatments when compared with plant community. 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. The long-term N deposition has greater effects on the understory plant community than on the soil microbial community and different conservation strategies should be considered.