12 Years of NPK Addition Diminishes Carbon Sink Potential of a Nutrient Limited Peatland

Peatlands store about a third of global soil carbon. Our aim was to study whether the vegetation feedbacks of nitrogen (N) deposition lead to stronger carbon sink or source in a nutrient limited peatland ecosystem. We investigated vegetation structure and ecosystem CO₂ exchange at Mer Bleue Bog, Canada, that has been fertilized for 7-12 years. We have applied 5 and 20 times ambient annual wet N deposition (0.8 g N m^-2) with or without phosphorus (P) and potassium (K). Gross photosynthesis, ecosystem respiration and net CO₂ exchange (NEE) were measured weekly during the growing season using chamber technique. Under the highest N(PK) treatments, the light saturated photosynthesis (PS_max) was reduced by 20-30% compared to the control treatment, whereas under moderate N and PK additions PS_max slightly increased or was similar to the control. The ecosystem respiration showed similar trends among the treatments, but changes in the rates were less pronounced. High nutrient additions led to up to 65% lower net CO₂ uptake than that in the control: In the NPK plots with cumulative N additions of 70, 19, and 0 g N m^-2, the daytime NEE in May-July 2011 averaged 0.8 (se. 0.3), 2.0 (se. 0.4), and 2.4 (se. 0.3) μmol m^-2 s^-1, respectively. In the N only plots with cumulative N additions of 45, 19, and 0 g N m^-2, the daytime NEE in May-July 2011 averaged 0.8 (se. 0.2), 2.6 (se. 0.4), and 1.8 (se. 0.3) μmol m^-2 s^-1, respectively. The reduced plant photosynthetic capacity and diminished carbon sink potential in the highest nutrient treatments correlated with the loss of peat mosses and were not compensated for by the increased vascular plant biomass that has mainly been allocated to woody shrub stems.