Seasonal and Spatial Variability of Biological N<sub>2</sub> Fixation in a Cool Temperate Bog
Tatjana Živković, Manuel Helbig, Tim R. Moore
Abstract
Abstract 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 15 N 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.