Photosynthetic Performance of Chamaedaphne calyculata after Twelve Years of Nutrient Addition at Mer Bleue Bog, Ontario, Canada
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Peatlands are important ecosystems in the global carbon cycle, as they store 30% of the world’s soil carbon. However, combustion of fossil fuels and fertilizer manufacture have increased reactive nitrogen in the form of nitrate and ammonium, which are limiting nutrients in many ecosystems including peatlands, and can cause cascading effects on these ecosystems. The fertilization experiment at Mer Bleue Bog in Ontario was set up in 2000 to investigate the impact of nutrient addition, including nitrogen (N), phosphorus (P) and potassium (K), on the carbon (C) sink function of this peatland. This study examined the effect of nutrient addition on the dominant ericaceous shrub at the bog, leatherleaf (Chamaedaphne calyculata), after twelve years of exposure to five and twenty times the growing season ambient wet N deposition rates with and without P and K. Leaf-level CO2 gas exchange and chlorophyll fluorescence, an indicator of plant stress in terms of light harvesting capacity, of both current-year and previous-year leaves were measured using a leaf chamber and Licor 6400 infrared gas analyzer to investigate the photosynthetic performance of this shrub. Additionally, from each leaf, maximum electron transport rate (Jmax) and Rubisco carboxylation rate (Vcmax) were derived from the CO2 response curve, and chlorophyll content analyzed. After twelve years of fertilization, nutrients were still being invested in chlorophyll and Rubisco, which are the main sinks of nutrients, especially N. Chlorophyll content also reflected differences between old and new leaves due to nutrient transport and light availability. Chlorophyll fluorescence ratios of all measured leaves were within the range of healthy leaves (0.75 to 0.83); therefore, there is no sign of plant stress in terms of maximum light harvesting capacity. New leaves had a significantly lower light harvesting capacity than old leaves due to physiological immaturity. Investment of nutrients both in light and dark reactions of photosynthesis did not translate into higher maximum gross photosynthetic rate (Pmax); in fact, net assimilation rates (Amax) were lowest in the highest nutrient treatments. The decreasing trend in Amax with increased nutrients was due to increased dark respiration along the treatment gradient, and this trend mirrored the response pattern of net ecosystem CO2 exchange, ecosystem respiration and gross ecosystem photosynthesis to nutrient addition. Thus, the shrub, in response to fertilization, contributes to a weaker C sink in the bog through increased respiration and unchanged photosynthesis.