Are Ombrotrophic Bog Shrubs Stressed under High Nutrient Load?

English Abstract:

The reactive nitrogen (N) assimilated by plants is usually invested in chlorophyll to improve light harvesting capacity and in soluble proteins such as Rubisco to enhance carbon (C) assimilation. We studied the effects of simulated atmospheric N deposition on two evergreen shrubs Chamaedaphne calyculata and Rhododendron groenlandicum in a nutrient-poor Mer Bleue Bog, Canada that has been fertilized with N as NO₃ and NH₄ (2-8 times ambient annual wet deposition) with or without phosphorus (P) and potassium (K) for 7-12 years. We examined how nutrient addition influences the plant performance at leaf and canopy level. We measured leaf level CO₂ exchange and chlorophyll fluorescence, an indicator of plant stress in terms of light harvesting capacity, and to study changes in photosynthetic nutrient use efficiency, we also determined the foliar chlorophyll, N, and P contents. At the canopy level, we examined growth responses and leaf longevity during two growing seasons. Regardless of treatment, the majority of leaves showed no signs of stress in terms of light harvesting capacity. The plants were N saturated: with increasing foliar N content, the higher proportion of N was not used in photosynthesis. Foliar net CO₂ assimilation rates did not differ significantly among treatments, but the additions of N, P, and K together resulted in higher respiration rates. The two shrubs had different strategies: C. calyculata was more responsive to nutrient additions, more deciduous-like at both leaf and canopy level, where as R. groenlandicum maintained evergreen features under nutrient load, shedding its leaves even later in the season. In all, simulated atmospheric N deposition did not benefit the photosynthetic apparatus of the dominant shrubs, but resulted in higher foliar respiration, contributing to stress and a weaker ecosystem C sink. Thus, elevated atmospheric deposition of nutrients to these systems may endanger C storage in peatlands.