Responses of Vegetation and Ecosystem CO2 Exchange to 9 Years of Nutrient Addition at Mer Bleue Bog

Anthropogenic nitrogen (N) loading has the potential to affect plant community structure and function, and the carbon dioxide (CO2) sink of peatlands. Our aim is to study how vegetation changes, induced by nutrient input, affect the CO2 exchange of a nutrient-limited bog. We conducted 9- and 4-year fertilization experiments at Mer Bleue bog, where we applied N addition levels of 1.6, 3.2, and 6.4 g N m(-2) a(-1), upon a background deposition of about 0.8 g N m(-2) a(-1), with or without phosphorus and potassium (PK). Only the treatments 3.2 and 6.4 g N m(-2) a(-1) with PK significantly affected CO2 fluxes. These treatments shifted the Sphagnum moss and dwarf shrub community to taller dwarf shrub thickets without moss, and the CO2 responses depended on the phase of vegetation transition. Overall, compared to the large observed changes in the vegetation, the changes in CO2 fluxes were small. Following Sphagnum loss after 5 years, maximum ecosystem photosynthesis (Pg(max)) and net CO2 exchange (NEEmax) were lowered (-19 and -46%, respectively) in the highest NPK treatment. In the following years, while shrub height increased, the vascular foliar biomass did not fully compensate for the loss of moss biomass; yet, by year 8 there were no significant differences in Pg(max) and NEEmax between the nutrient and the control treatments. At the same time, an increase (24-32%) in ecosystem respiration (ER) became evident. Trends in the N-only experiment resembled those in the older NPK experiment by the fourth year. The increasing ER with increasing vascular plant and decreasing Sphagnum moss biomass across the experimental plots suggest that high N deposition may lessen the CO2 sink of a bog.