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

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Moss-associated nitrogen (N2) fixation performed by epiphytic, N2-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 N2 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 N2 fixation in mosses over 8 weeks and assessed their nutrient loss (fixed N2, total dissolved N, ammonium, phosphate) during the FTCs. We expected lower nitrogenase activity in mosses exposed to more severe FTCs and different sensitivities of N2 fixation towards FTCs along the climate gradient. However, no differences were found in N2 fixation between mild and severe FTCs, but N2 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 N2 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.

Original languageEnglish
Article number103796
JournalActa Oecologica
Volume113
Number of pages10
ISSN1146-609X
DOIs
Publication statusPublished - Nov 2021

Bibliographical note

CENPERMOA[2021]
Funding Information:
We thank Maja H. Wahlgren for assistance with the ethylene analyses, and Karina E. Clemmensen for assistance with collection of mosses at Fiby Urskog. We thank Karina E. Clemmensen, Klaus S. Larsen and Teis N. Mikkelsen for providing climate data. Funding was provided by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 947719 to KR), by the Independent Research Fund Denmark (IRFD) Sapere Aude Grant (Grant id: 7027-00011B to KR) and the Danish National Research Foundation (Center for Permafrost, CENPERM DNRF100).

Publisher Copyright:
© 2021 The Authors

    Research areas

  • Acetylene reduction, Climate change, Cyanobacteria, Diazotrophs, Nutrient limitation, Phosphorus

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