Microbial nitrogen fixation and methane oxidation are strongly enhanced by light in Sphagnum mosses

Peatlands have acted as C-sinks for millennia, storing large amounts of carbon, of which a significant amount is yearly released as methane (CH₄). Sphagnum mosses are a key genus in many peat ecosystems and these mosses live in close association with methane-oxidizing and nitrogen-fixing microorganisms. To disentangle mechanisms which may control Sphagnum-associated methane-oxidation and nitrogen-fixation, we applied four treatments to Sphagnum mosses from a pristine peatland in Finland: nitrogen fertilization, phosphorus fertilization, CH₄ addition and light. N and P fertilization resulted in nutrient accumulation in the moss tissue, but did not increase Sphagnum growth. While net CO₂ fixation rates remained unaffected in the N and P treatment, net CH₄ emissions decreased because of enhanced CH₄ oxidation. CH₄ addition did not affect Sphagnum performance in the present set-up. Light, however, clearly stimulated the activity of associated nitrogen-fixing and methane-oxidizing microorganisms, increasing N₂ fixation rates threefold and CH₄ oxidation rates fivefold. This underlines the strong connection between Sphagnum and associated N₂ fixation and CH₄ oxidation. It furthermore indicates that phototrophy is a strong control of microbial activity, which can be directly or indirectly.