Background/Question/Methods
Atmospheric nitrogen deposition may have serious implications for the species composition, primary production, and carbon dioxide exchange in northern peatlands. Our previous results after five years of fertilization at the Mer Bleue Bog indicated that high N (+PK) supply may reduce net ecosystem carbon uptake as a result of reductions in ecosystem gross photosynthesis (Pg). The present study examines possible mechanisms for this photosynthetic draw down by measuring leaf photosynthesis rates and morphology of the three dominant dwarf ericaceous shrubs including the deciduous Vaccinium myrtilloides, and evergreen Chamadaphane calyculata and Ledum groenlandicum. Treatments included low (1.6g N y^-1) and high (6.4g N y^-1) N with and without PK. We measured leaf photosynthesis for individual leaves and calculated Vcmax in order to estimate the effects of fertilization on Rubisco enzyme activity. Morphometric measurements included length, width, thickness, area, mass and specific leaf area (SLA, cm²/g).
Results/Conclusions

Species responded differently to fertilization in Vcmax as well as morphology. Ledum groenlandicum had significantly higher Vcmax in low N and low NPK but not high N or high NPK treatments. V. myrtilloides had significantly lower Vcmax in low N plots, but no response in the other treatments. C. calyculata did not respond significantly to treatment. Of the three species, V. myrtilloides had the lowest Vcmax values in all treatments. Differences between Vcmax values for these species were smaller in high-N and high NPK plots than in controls. Morphology of L. groenlandicum and V. myrtilloides leaves showed significant responses to treatments. V. myrtilloides increased in leaf area in high-NPK treatments and L. groenlandicum increased in SLA in low NPK treatments. Treatments did not affect C. calyculata leaf morphology. The differences in species responses may be explained by leaf nitrogen concentrations. None of the species measured showed significant increases in Vcmax in the high N or NPK treatments, but moderate N addition increased photosynthetic capacity of L. groenlandicum. These preliminary results indicate that species at Mer Bleue may be nitrogen-saturated with high-N and NPK treatments.