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 (CH(4)). 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, CH(4) addition and light. N and P fertilization resulted in nutrient accumulation in the moss tissue, but did not increase Sphagnum growth. While net CO(2) fixation rates remained unaffected in the N and P treatment, net CH(4) emissions decreased because of enhanced CH(4) oxidation. CH(4) 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 N(2) fixation rates threefold and CH(4) oxidation rates fivefold. This underlines the strong connection between Sphagnum and associated N(2) fixation and CH(4) oxidation. It furthermore indicates that phototrophy is a strong control of microbial activity, which can be directly or indirectly.