Northern peatland bogs are major global carbon sinks, yet enhanced nitrogen deposition threatens their long-term terrestrial carbon storage. Here, we evaluated the effects of long-term additions (2012-2025) of 0.0 (controls), 1.6, 3.2, and 6.4 g N m⁻² yr⁻¹ of either NH4Cl or NaNO3 on plant communities and peat biogeochemistry at Mer Bleue Bog (Ottawa, Canada). Sphagnum moss cover declined sharply at 1.6 g N m⁻² yr⁻¹ under both treatments and was entirely lost at 3.2 and 6.4 g N m⁻² yr⁻¹. Additionally, leaf litter dominated the peat surface, approaching 100% cover in the 6.4 g N m⁻² yr⁻¹ plots, while Vaccinium species cover increased. This resulted in the collapse of the peat column, where average depth to water-table from the peat surface decreased from 40 cm (controls) to 26 cm (all treatments). Plant root simulator (PRS) probes showed that NH4+-N persisted in the NH4Cl treatment plots, while NO3--N was near absent in the NaNO3 treatment plots, even at 6.4 g m⁻² yr⁻¹. Carbon stock did not significantly vary between treatments and control plots whereas nitrogen stock was significantly higher in treatment plots, exclusively in the 0-5 cm layer. pH varied drastically in the top 15 cm of peat where NO3--N plots had values of ~ 6.0 while NH4+-N plot values were ~ 3.8 (controls were ~ 4.2). Our results show that elevated rates of N deposition can drive major changes in plant cover and peat physical and biogeochemical properties.