Variation in symbiotic N(2) fixation rates among Sphagnum mosses

Biological nitrogen (N) fixation is an important process supporting primary production in ecosystems, especially in those where N availability is limiting growth, such as peatlands and boreal forests. In many peatlands, peat mosses (genus Sphagnum) are the prime ecosystem engineers, and like feather mosses in boreal forests, they are associated with a diverse community of diazotrophs (N(2)-fixing microorganisms) that live in and on their tissue. The large variation in N(2) fixation rates reported in literature remains, however, to be explained. To assess the potential roles of habitat (including nutrient concentration) and species traits (in particular litter decomposability and photosynthetic capacity) on the variability in N(2) fixation rates, we compared rates associated with various Sphagnum moss species in a bog, the surrounding forest and a fen in Sweden. We found appreciable variation in N(2) fixation rates among moss species and habitats, and showed that both species and habitat conditions strongly influenced N(2) fixation. We here show that higher decomposition rates, as explained by lower levels of decomposition-inhibiting compounds, and higher phosphorous (P) levels, are related with higher diazotrophic activity. Combining our findings with those of other studies, we propose a conceptual model in which both species-specific traits of mosses (as related to the trade-off between rapid photosynthesis and resistance to decomposition) and P availability, explain N(2) fixation rates. This is expected to result in a tight coupling between P and N cycling in peatlands.