Seasonal and Spatial Variability of Biological N2 Fixation in a Cool Temperate Bog

Abstract

AbstractNorthern 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 (N2) 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 N2 fixation across a hydrological transect (hummock‐hollow‐beaver pond edge) in a temperate ombrotrophic bog. We measured N2 fixation in live Sphagnum plants by acetylene reduction assay calibrated with a 15N2 tracer method, bi‐weekly, from May to November over two growing seasons. We found that N2 fixation increased with soil temperature at 5 cm in the living Sphagnum mat explaining the seasonal variability in N2 fixation. Peak N2 fixation rates occur in mid‐August, when N2 fixation rates are about 10 times larger than during the shoulder seasons (May and November). Spatially, N2 fixation was larger in wetter Sphagnum with larger gravimetric water content in Sphagnum. This relationship was most pronounced in the peak growing season when N2 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 N2 fixation, which accounts for about a fourth of the N accumulated annually into Sphagnum. Future contributions from Sphagnum‐associated N2 fixation to N budgets in peatlands will depend on temperature and moisture changes which have contrasting effects on N2 fixation rates.