N2-fixation by methanotrophs sustains carbon and nitrogen accumulation in pristine peatlands

Symbiotic relationships between N-2-fixing prokaryotes and their autotrophic hosts are essential in nitrogen (N)-limited ecosystems, yet the importance of this association in pristine boreal peatlands, which store 25 % of the world's soil (C), has been overlooked. External inputs of N to bogs are predominantly atmospheric, and given that regions of boreal Canada anchor some of the lowest rates found globally (1 kg N ha(-1) year(-1)), biomass production is thought to be limited primarily by N. Despite historically low N deposition, we show that boreal bogs have accumulated approximately 12-25 times more N than can be explained by atmospheric inputs. Here we demonstrate high rates of biological N-2-fixation in prokaryotes associated with Sphagnum mosses that can fully account for the missing input of N needed to sustain high rates of C sequestration. Additionally, N amendment experiments in the field did not increase Sphagnum production, indicating that mosses are not limited by N. Lastly, by examining the composition and abundance of N-2-fixing prokaryotes by quantifying gene expression of 16S rRNA and nitrogenase-encoding nifH, we show that rates of N-2-fixation are driven by the substantial contribution from methanotrophs, and not from cyanobacteria. We conclude biological N-2-fixation drives high sequestration of C in pristine peatlands, and may play an important role in moderating fluxes of methane, one of the most important greenhouse gases produced in peatlands. Understanding the mechanistic controls on biological N-2-fixation is crucial for assessing the fate of peatland carbon stocks under scenarios of climate change and enhanced anthropogenic N deposition.