Long-term changes and impacts of nutrient addition to vegetation and CO2 fluxes at ombrotrophic Mer Bleue Bog, Canada

To study long-term impacts of nutrient addition on carbon sequestration capacity, we investigated changes in vegetation and ecosystem CO₂ exchange at Mer Bleue Bog, Canada in plots that had been fertilized with nitrogen (N) or with N plus phosphorus (P) and potassium (K) and in non-fertilized control plots for 13-18 years. The vegetation structure and species composition were measured in all treatments mid July 2001-2018 (14 measurement years) using a point intercept method. Gross photosynthesis, ecosystem respiration, and net CO₂ exchange were measured weekly during June–August 2001-2016 (7 measurement years, usually every two years) using climate-controlled chambers. Using Bayesian approach, we analyzed whether there were changes over time in vegetation and ecosystem CO₂ exchange and whether those trends differed between treatments. We found that shrubs had become taller and more abundant at the unfertilized plots during the 18 study years likely owing to warmer summers and a drying trend that favor shrubs. At the fertilized plots, the increase in shrub height was greater and faster than in unfertilized plots, and the addition of PK with N further accelerated growth of the shrub canopy. Among the dwarf shrubs, only Chamaedaphne calyculata benefitted from the fertilization. No change towards more gramineous vegetation was observed. Because the plants at the bog are N-P co-limited rather than N-limited, PK addition alleviated growth limitation. Sphagnum cover decreased with the increasing nutrient load. Ecosystem respiration increased in all treatments, but it increased faster and more in fertilized plots than in unfertilized plots. In all treatments, increases in ecosystem respiration resulted in less net CO₂ uptake during the recent ten years (since 2008), because gross photosynthesis rates did not compensate for increases in ecosystem respiration. In general, the magnitude of this trend of reduced net C sink potential did not differ markedly in unfertilized from fertilized plots. These CO₂ flux trends could be explained by changes in nutrient availability, a larger proportion of nongreen biomass in dense stands and enhanced peat decomposition. Our long-term field experiment revealed that ecosystem responses to the combination of nutrient addition and drying must be considered when evaluating the impact of climate change on the carbon sink potential of peatlands.