Seasonal and Spatial Variability of Biological N-2 Fixation in a Cool Temperate Bog

Northern peatlands are globally important carbon (C) and nitrogen (N) sinks due to slow decomposition rates resulting in long-term organic matter accumulation. Despite their large N storage, peatlands depend on sources of bio-available N to sustain their biomass production. Di-nitrogen (N-2) fixation represents an important biological N source in ombrotrophic bogs, but its environmental controls are still poorly understood. We examined seasonal and spatial variability of Sphagnum-associated N-2 fixation across a hydrological transect (hummock-hollow-beaver pond edge) in a temperate ombrotrophic bog. We measured N-2 fixation in live Sphagnum plants by acetylene reduction assay calibrated with a N-15(2) tracer method, bi-weekly, from May to November over two growing seasons. We found that N-2 fixation increased with soil temperature at 5 cm in the living Sphagnum mat explaining the seasonal variability in N-2 fixation. Peak N-2 fixation rates occur in mid-August, when N-2 fixation rates are about 10 times larger than during the shoulder seasons (May and November). Spatially, N-2 fixation was larger in wetter Sphagnum with larger gravimetric water content in Sphagnum. This relationship was most pronounced in the peak growing season when N-2 fixation rates were the highest. Finally, we estimated that the Mer Bleue bog receives around 0.3 g N m(-2) annually through Sphagnum-associated N-2 fixation, which accounts for about a fourth of the N accumulated annually into Sphagnum. Future contributions from Sphagnum-associated N-2 fixation to N budgets in peatlands will depend on temperature and moisture changes which have contrasting effects on N-2 fixation rates.