Extreme freeze-thaw cycles do not affect moss-associated nitrogen fixation across a temperature gradient, but affect nutrient loss from mosses.

Moss-associated nitrogen (N 2) fixation performed by epiphytic, N 2 -fixing bacteria (diazotrophs) contributes significantly to ecosystem N input in pristine habitats. While we have some understanding of the effects of climate warming on moss-associated N 2 fixation, we lack data on effects of freeze-thaw cycles (FTCs) on diazotroph activity, although increased frequency of FTCs is predicted. We collected the widespread moss Pleurozium schreberi along a climate gradient (temperate, boreal, arctic) and exposed moss and associated diazotrophs to severe (20 °C difference, cycling between +10 and −10 °C) and mild (6 °C difference, ±3 °C) diurnal FTCs. We measured N 2 fixation in mosses over 8 weeks and assessed their nutrient loss (fixed N 2 , total dissolved N, ammonium, phosphate) during the FTCs. We expected lower nitrogenase activity in mosses exposed to more severe FTCs and different sensitivities of N 2 fixation towards FTCs along the climate gradient. However, no differences were found in N 2 fixation between mild and severe FTCs, but N 2 fixation in mosses from the temperate heath was less susceptible to FTCs than those from colder sites, suggesting adapted temperate diazotroph communities. Mosses lost little N, most at constant, positive temperatures, while more phosphate was lost from mosses exposed to FTCs, depending on the positioning along the climate gradient, mirroring nutrient demand and limitation. Our results show that moss-associated N 2 fixation is less susceptible towards FTCs than expected but nutrient loss from moss carpets can increase following FTCs, with consequences for nutrient pools and fluxes. [Display omitted] • Nitrogen fixation by moss-associated cyanobacteria is strongly moisture dependent. • We assessed the effects of mild and severe freeze-thaw cycles (FTCs) on N fixation. • Activity was not consistently affected by FTCs across climates. • But substantial amounts of nutrients were lost from the moss after repeated FTCs. [ABSTRACT FROM AUTHOR]