The C:N:P:K stoichiometry in an ombrotrophic peatland

Ombrotrophic peatlands are nutrient-deficient systems and important carbon (C) sinks, yet the stoichiometry of nitrogen (N), phosphorus (P) and potassium (K), essential for plant growth and decomposition, has rarely been studied in these systems. Under contemporary anthropogenic environmental changes, such as enhanced atmospheric deposition, the balance among multiple elements during ecological processes and interactions, i.e. ecological stoichiometry, is altered. The stoichiometry in plants and soils in peatlands and the response of these stoichiometric characteristics to nutrient enrichment are critical in a world of elemental imbalance between organisms and their environment. The objectives of this research were to examine the seasonal variation in C:N:P:K stoichiometry among different plant functional types (PFTs), the effects of nutrient addition on nutrient resorption and the degree of stoichiometric homeostasis, and the decomposition cascade of C:N:P stoichiometry from mature tissues to peat at Mer Bleue Bog, eastern Ontario, Canada. Along with a greater variation in nutrient concentrations and stoichiometric ratios among PFTs than across the growing season, a convergence of C:N:P:K to a mass ratio of 445:14:1:9 was found at peak growing season, indicating N and P co-limitation. A substantial resorption of N, P and K during leave senescence was observed and was significantly different between the two dominant shrub species, Chamaedaphne calyculata and Rhododendron groenlandicum. The interaction between N, P and K addition on their resorption reflected the stoichiometric coupling of nutrient cycling. In response to long-term N, P and K fertilization, shrubs and mosses showed strong and weak homeostasis, respectively; whereas the dominance of shrubs with strong homeostasis did not enhance the stability or the productivity of the plant community. The examination of the decomposition cascade of C:N:P stoichiometry confirmed the resorption-associated decrease in C:N and C:P ratios from mature to senescent tissues. A possible mechanism related to plant/mycorrhizae uptake of P may account for the increase in C:P and N:P ratios in peat. This thesis contributes new knowledge on the stoichiometry of plants and soils in an ombrotrophic peatland. Specifically, it emphasizes the important role of P in peatland biogeochemical cycles, especially the plant and mycorrhizal uptake of P in these nutrient-deficient ecosystems. The substantial variation in nutrient resorption and stoichiometric homeostasis between species and among PFTs shed new light on the potential shifts in plant community composition under environmental changes.

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