Many moss species are associated with nitrogen (N)-fixing bacteria (diazo-trophs) that support the N supply of mosses. Our knowledge relates primar-ily to pristine ecosystems with low atmospheric N input, but knowledge ofbiological N fixation (BNF) and diazotrophic communities in mosses in tem-perate forests with high N deposition is limited. We measured BNF ratesusing the direct stable isotope method and studied the total and potentiallyactive diazotrophic communities in two abundant mosses,BrachytheciumrutabulumandHypnum cupressiforme, both growing on lying deadwoodtrunks in 25 temperate forest sites. BNF rates in both mosses were similarto those observed in moss species of pristine ecosystems.H. cupressiformefixed three times more N2and exhibited lower diazotrophicrichness thanB. rutabulum.Frankiawas the most prominent diazotroph fol-lowed by cyanobacteriaNostoc.Manganese, iron, and molybdenum con-tents in mosses were positively correlated with BNF and diazotrophiccommunities.Frankiamaintained high BNF rates inH. cupressiformeandB. rutabulumeven under high chronic N deposition in Central European for-ests. Moss N concentration and15N abundance indicate a rather minor con-tribution of BNF to the N nutrition of these mosses.