Abstract: To analyze the impacts of increased nutrient availability on plant community structure, we conducted a long-term field N and P addition experiment in a typical peatland in the north of Greater Khingan Mountain and investigated species dominance, community diversity, and aboveground biomass after four, six, and eight years of N (6 g N·m^-2·a^-1) and P (2 g P·m^-2·a^-1) addition. The results showed that P addition did not affect the dominances of deciduous shrub, evergreen shrub, sedge, grass, forb, moss, and lichen. However, N addition significantly increased the importance values and dominances of deciduous shrub and grass but reduced the importance values and dominances of moss and lichen. The magnitude of N addition effects increased with elevating experimental duration. Both N and P addition decreased species richness, diversity, and evenness. The N addition-induced decline in species diversity primarily resulted from the disappearance of evergreen shrub, moss, and lichen, whereas decreased species diversity under P addition were mainly caused by the loss of forb and moss. Phosphorus addition did not affect aboveground biomass of plants, while N addition significantly reduced aboveground biomass of evergreen shrub, moss, and lichen and increased that of deciduous shrub and grass. Nitrogen and P addition produced a significant interaction on aboveground plant biomass in this community. Nitrogen addition and combined N and P addition treatments had 87% and 128% greater aboveground plant biomass than the control, respectively. These results suggested that nutrient enrichment could promote plant growth but induce reductions in species diversity, and such effect would be amplified by increasing experimental duration.

Key words: boreal peatland, nitrogen deposition, nutrient enrichment, species diversity, plant biomass