Methanotrophy induces nitrogen fixation during peatland development

Abstract

Nitrogen (N) accumulation rates in peatland ecosystems indicate significant biological atmospheric N2 fixation associated with Sphagnum mosses. Here, we show that the linkage between methanotrophic carbon cycling and N2 fixation may constitute an important mechanism in the rapid accumulation of N during the primary succession of peatlands. In our experimental stable isotope enrichment study, previously overlooked methane-induced N2 fixation explained more than one-third of the new N input in the younger peatland stages, where the highest N2 fixation rates and highest methane oxidation activities co-occurred in the water-submerged moss vegetation.

Keywords: CH4; diazotrophy; mire; peat; phosphorus.

Fig. 1.

Carbon (left axis downward) and nitrogen (right axis downward) accumulation rates during the 2,500-y primary succession of Siikajoki peatlands from meadows, mesotrophic fens, and oligotrophic fens toward fen-bog transitions on the land-uplift coast of the Bothnian Bay (age cohorts SJ2–SJ5; ref. 23). The error bars are SD. The estimates of site age means are based on the land-uplift rate (meadows and mesotrophic fens) or on radiocarbon dating (oligotrophic fens and fen-bog transitions); m a.s.l., meters above sea level; BP, before present.

Fig. 2.

Sphagnum moss-associated N₂ fixation and CH₄ oxidation (biomass incorporation of CH₄-derived C) in 12 peatlands of the peatland chronosequence (meadows, mesotrophic and oligotrophic fens, and fen-bog transitions) based on the stable isotope pulse-labeling experiment. The error bars are SEM (n = 3 peatlands in each stage). (A) Contribution of dark (heterotrophic, black), light-induced (phototrophic, white), and CH₄-induced (methanotrophic, cross-hatched) N₂ fixation in flark (F) and hummock (H) vegetation. The meadows have only flark Sphagnum. (B) The contribution of CH₄-induced N₂ fixation (cross-hatched) to Sphagnum-associated N₂ fixation. Data are averages of light and dark incubations in ¹⁵N₂ and ¹⁵N₂+¹³CH₄ treatments, respectively, weighted with the proportions of flark and hummock microhabitats in each successional stage. Long-term average peat N accumulation rates for the sites (total accumulation divided by the year since peatland initiation; ref. 23) is shown with filled circles. (C) Incorporation of ¹³CH₄-C into the biomass (moss + microbes) based on the weighted averages of flark and hummock microhabitats in each successional stage. In each bar, the filled area indicates the incorporation of ¹³CH₄-derived C in the dark (i.e., incorporation of CH₄ into methanotroph biomass) and the open area indicates the average additional incorporation of ¹³CH₄-derived C under prevailing light conditions (i.e., incorporation of CO₂ emitted by methanotrophs into autotrophic plant or microbial biomass via photosynthesis).

Fig. 3.

The relationships between the rates of N₂ fixation, CH₄ oxidation and phosphorus content in Sphagnum moss samples of different successional stages of peatlands (●, meadow; ■, mesotrophic fen; ▲, oligotrophic fen, and ▼, fen-bog transition). (A) The relationship between N₂ fixation (ln transformed) and moss P content (ln N₂ fixation = 1.25 ± 0.14[P content]+0.36 ± 0.13 (coefficient ± SE), R² = 0.49, df _{reg, res} 1,82; Table S5). The moss N:P ratios >16 indicating P limitation (32) are shown with open symbols. Averages for all four treatments per microhabitat and stage are shown (n = 84 incubations). (B) The Spearman rank correlation between the rates of CH₄ oxidation (measured as biomass incorporation of CH₄-derived C) and CH₄-induced N₂ fixation (calculated as the difference in N₂ fixation between ¹⁵N₂+¹³CH₄ and ¹⁵N₂ only treatments). The moss P content <0.5 mg⋅g⁻¹ is indicated by open symbols (n = 42 incubations).

Fig. 4.

The interplay of N₂-fixing bacteria in the upper moss vegetation. Phototrophic, heterotrophic, and methanotrophic bacteria all take part in the fixation of atmospheric N₂ to plant available N (NH₄⁺). The supply of CH₄ may support the direct methanotrophic N₂ fixation, but it may also provide additional CO₂ for phototrophic microbes, as well as for the moss itself. Both phototrophs and methanotrophs provide organic molecules (CH₂O) to heterotrophic N₂ fixers. Methanotrophic activity may also decrease the oxygen tension, providing a more suitable microenvironment for the oxygen-sensitive nitrogenase enzyme complex.