Biological nitrogen fixation, diversity and community structure of diazotrophs in two mosses in 25 temperate forests

Many moss species are associated with nitrogen (N)-fixing bacteria (diazotrophs) that support the N supply of mosses. Our knowledge relates primarily to pristine ecosystems with low atmospheric N input, but knowledge of biological N fixation (BNF) and diazotrophic communities in mosses in temperate forests with high N deposition is limited. We measured BNF rates using the direct stable isotope method and studied the total and potentially active diazotrophic communities in two abundant mosses, Brachythecium rutabulum and Hypnum cupressiforme, both growing on lying deadwood trunks in 25 temperate forest sites. BNF rates in both mosses were similar to those observed in moss species of pristine ecosystems. H. cupressiforme fixed three times more N2 and exhibited lower diazotrophic richness than B. rutabulum. Frankia was the most prominent diazotroph followed by cyanobacteria Nostoc. Manganese, iron, and molybdenum contents in mosses were positively correlated with BNF and diazotrophic communities. Frankia maintained high BNF rates in H. cupressiforme and B. rutabulum even under high chronic N deposition in Central European forests. Moss N concentration and 15N abundance indicate a rather minor contribution of BNF to the N nutrition of these mosses. Biological nitrogen (N) fixation of two common mosses, Brachythecium rutabulum and Hypnum cupressiforme in temperate forests were high despite chronically high N deposition. H. cupressiforme obtains more N than B. rutabulum but has fewer active bacteria. The majority of moss bacteria belong to Frankiaand to Nostoc. Moss N content correlates with manganese, iron and molybdenum. This study emphasizes the role of diverse bacteria in mosses' N acquisition despite chronic N deposition and its significance for ecological preservation.image