Nitrogen fixation in Sphagnum mosses is affected by moss species and water table level

Abstract

Nitrogen (N2) fixation by moss-associated bacteria is an important N source in boreal peatlands and forests. Here we studied whether moss species, water table fluctuations, methane (CH4) availability and diazotroph community structure would affect the rate of Sphagnum-associated N2 fixation. Diazotrophy and methanotrophy were studied in parallel in a double labeling (15 N2 and 13CH4) experiment in forest and fen habitats. The role of N2-fixing methanotrophs was further characterized by the phylogenetic analysis of nifH genes encoding for dinitrogenase reductase. N2 fixation rates were dependent on the moss species in a habitat level, but independent of the diazotroph community structure. Only 6 % of the nifH sequences were taxonomically assigned to cyanobacteria, while the majority (82 %) of genes were assigned to Alphaproteobacteria clustering with the nifH sequences of the order Rhizobiales, but without close matches to sequences of cultivated species. In the originally submerged fen mosses, water increased both N2 fixation and CH4 oxidation rates. However, such effect was not seen in forest mosses grown above the water table level. CH4 addition did not enhance N2 fixation. N2 fixation associated to Sphagnum mosses was controlled by the moss species and the water environment. Although most of the observed nifH sequences were related to the order Rhizobiales, methanotrophs were not responsible for the N2 fixation.