Vegetation Composition in Bogs is Sensitive to Both Load and Concentration of Deposited Nitrogen: A Modeling Analysis

Changes in vegetation composition due to nitrogen deposition may influence the role of peatlands in the carbon cycle. The impact of nitrogen (N) on vegetation composition of the Mer Bleue Bog (Ontario) and the underlying mechanisms were studied using a coupled carbon (C) and nitrogen wetland model (PEATBOG). The model was applied to data from a long-term nutrient fertilization experiment and the results were compared with the observed C and N pools in plants, peat, and soil water after 8 years of fertilization with 1.6, 3.2, and 6.4 gN m(-2) y(-1) and additional phosphorus and potassium. The evaluated model was employed to simulate the vegetation dynamics in the peatland fertilized with different loads and concentrations of N. The model suggested a shift in plant functional types from moss-shrub dominated to graminoid dominated with increasing N load. Suppression of mosses by N fertilization was mitigated by daily deposition of N at a low concentration. Mosses became extinct in the simulations only when the concentration and load of N deposition were high, corroborating empirical results from the fertilization experiment. Tracking the deposited N in the system, the model indicated that it was primarily sequestered in the peat. Dissolved N concentrations in peat and export from the bog increased with N load and decreased with N concentration, leading to higher N uptake by vascular plants. The results further suggested that the observed detrimental effect of N on mosses was most likely due to toxicity caused by N uptake exceeding N assimilation. Exposure of mosses to high N concentrations in precipitation and evaporating water on moss tissues is thus a factor that needs attention when considering critical loads of N deposition on peatlands.