Methanotrophic N2-Fixation in Boreal Peatlands: Master Regulation of Newly Fixed N and Moderation of CH4 Fluxes to the Atmosphere

Abstract:

Boreal peatlands are important to global carbon (C) and nitrogen (N) cycling. While they cover only 3-4% of the terrestrial surface, they account for 25-30% and 9-15% of the world’s soil C and N, respectively. Globally, peatlands function as a net sink for atmospheric CO₂, but also act as a net source of CH₄. In peatlands of Alberta, Canada, rates of atmospheric N deposition are low: ~1 kg·ha^-1·yr^-1, however, NPP of Sphagnum mosses is surprisingly high. Sphagnum mosses are able to maintain high levels of NPP due to their symbiotic relationship with N₂-fixing methanotrophs. Annually, rates of N₂-fixation typically provide between 10-30 kg of newly fixed N per ha, while also oxidizing CH₄. CH₄ fluxes from boreal peatlands in Canada are typically quite low, however, fluxes also can be high during rare episodic ebullition events. CH₄ accumulation and storage is substantial, especially at depth as CH₄ concentrations in peat pore water typically exceed 1000 µM at 1-m below the peat surface. Given that CH₄ fluxes are typically low, we wanted to examine the importance of CH₄ oxidation in these same peats. We hypothesized that methanotrophic N₂-fixation may be important not only in providing inputs of newly fixed N, but also in regulating CH₄ emissions from peatlands. We measured concentrations of CH₄ in peat porewaters, CH₄ flux rates, CH₄ oxidation rates, biological N₂-fixation rates. Porewater CH₄ concentrations were highest at depth and decreased to negligible amounts closer to the surface of the water table. CH₄ flux rates were low and ranged from -2.6 to +8.7 µmol·m^-2·min^-1 (negative values indicate net CH₄ consumption). In Sphagnum incubations, rates of CH₄ oxidation (0.6-10.2 mmol·m^-2·min^-1) were ~2-3 orders of magnitude higher than CH₄ flux rates. Additionally, N₂-fixation rates in paired cores ranged from 11.7-63.3 kg·ha^-1·yr^-1; CH₄ oxidation rates in these same units ranged from ~3.1-622 kg·CH₄ oxidized ha^-1·yr^-1. Additionally, we confirmed that N₂-fixing methanotrophs (not cyanobacteria) were the dominant fixers by quantifying gene expression of nifH, the gene that encodes for N₂-fixation. Collectively, these data confirm that N₂-fixing methanotrophs both provide inputs of newly fixed N, and substantially moderate CH₄ fluxes from boreal peatlands of Alberta to the atmosphere.