Plant Response to Fertilization in a Cool Temperate Peatland
Peatlands are nutrient-limited ecosystems. Human activities are causing an increase in nitrogen (N) deposition, which may lead to fertilization of bogs and alter vascular plant densities and biomass. N deposition affects ecosystem function, and potentially alters the system s ability to sequester carbon. In the summer of 2005 we measured this effect in an ombrotrophic bog, Mer Bleue, near Ottawa, Canada with a fertilization experiment established in 2000. We measured leaf-level CO2 exchange with a LI-6400 portable photosynthesis system. We used these data to calculate the maximum rate of photosynthetic capacity (Vmax) between the high fertilization (20NPK, 20 times the ambient summer N deposition, or 6.4 g N m-2 as NH4NO3, and 6.3 g P m-2, 5.0 g K m-2 as KH2PO4) treatment plots and control plots. We quantified above ground vascular plant biomass through non-destructive measurements of stem height and stem number within the 0.6 x 0.6m quadrat where we measured net ecosystem CO2 exchange. We destructively measured shrub biomass, number of leaves, leaf size, number of stems, C: N ratio of the leaves, and stem length for clipped plant samples collected from outside the CO2 measurement quadrats. We also measured leaf area index, the mass of litter and litter cover with in the 0.6x 0.6m quadrats. After five years of nutrient addition, above ground biomass of shrubs significantly increased between the control and high fertilization plots (20NPK). This pattern is perhaps explained by the increase in both stem length and leaf area with the fertilizer addition. A decrease in C: N ratio suggests that plants in the fertilizer treatments are taking up the added nutrients. However, an important difference was found in the leaf level photosynthesis data, which showed a significant decrease in Vmax between the control and the high fertilization treatment. These results have important implications for the ecosystem response to environmental changes. The increase in biomass and litter production of vascular plants will have effects on carbon storage as a result of the decomposability of this matter. The increase in biomass may be offset by decreases in leaf-level photosynthesis, potentially altering the carbon uptake within the system.