What drives biological nitrogen ﬁxation in high
arctic tundra: Moisture or temperature?
Department of Biology, Terrestrial Ecology Section, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen, Denmark
Center for Permafrost (CENPERM), University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark
Citation: Rousk, K., P. L. Sorensen, and A. Michelsen. 2018. What drives biological nitrogen ﬁxation in high arctic
tundra: Moisture or temperature? Ecosphere 9(2):e02117. 10.1002/ecs2.2117
Biological nitrogen (N
) ﬁxation is one of the main sources of available N for pristine ecosys-
tems such as subarctic and arctic tundra. Although this has been acknowledged more than a decade ago,
few attempts have been undertaken to identify the foremost driver of N
ﬁxation in the high Arctic. Here,
we report results from in situ measurements of N
ﬁxation throughout the main growing period (June–
August) in high arctic tundra, Greenland, in climate change treatments, shading and warming, and control.
Nitrogen ﬁxation was also measured in cores that received additional water prior to the measurements.
The climate change ﬁeld treatments did not lead to signiﬁcant changes in any measured parameters; how-
ﬁxation was promoted by adding water, and moisture was the most important factor inﬂuencing
ﬁxation in all climate change ﬁeld treatments. Maximum N
ﬁxation rates were measured below 14°C
soil temperature, which is much lower than the theoretical and previously reported temperature optimum
for the nitrogenase enzyme. Diazotroph (N
ﬁxing bacteria) communities are adapted to low temperatures
in high arctic settings, and increased temperature in a future climate may lead to decreased N
rates, or to a shift in diazotroph communities.
Key words: arctic; bryophytes; climate change; cyanobacteria; heath tundra; nitrogen ﬁxation; nitrogenase.
Received 3 October 2017; revised 20 January 2018; accepted 24 January 2018. Corresponding Editor: Yude Pan.
Copyright: © 2018 Rousk et al. This is an open access article under the terms of the Creative Commons Attribution
License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
Nitrogen is usually the primary limiting nutri-
ent for plant growth in subarctic and arctic tun-
dra (Shaver and Chapin 1980, Michelsen et al.
2012). While atmospheric N deposition is one
major source of plant available N in many
ecosystems, N deposition in pristine ecosystems,
such as subarctic and arctic tundra, is low (<2kg
; Van Cleve and Alexander 1981,
nuelas et al. 2013) and is likely not sufﬁcient to
cover plant-N demand. Here, ﬁxation of atmo-
is a large source of plant available N
and is performed by free-living N
(diazotrophs), and diazotrophs associated with
lichens and mosses (Hobara et al. 2006, Rousk
et al. 2016). In boreal forests, subarctic and arctic
tundra, moss-associated N
ﬁxation can exceed
N deposition rates, mostly due to the high cover-
age and biomass of mosses (Van Cleve et al.
1983, Turetsky 2003, Gavazov et al. 2010, Michel-
sen et al. 2012). However, N
ﬁxation is limited
by the availability of molybdenum and phospho-
rus (Rousk et al. 2017a), as well as by extreme
temperatures (Gundale et al. 2012, Stewart et al.
2014) and low moisture conditions (Rousk et al.
2014, Stewart et al. 2014). While it has been
shown in laboratory (Rousk et al. 2014, 2017b),
ﬁeld (Rousk et al. 2015), and modeling (Rousk
and Michelsen 2017) approaches that moisture is
the major factor driving moss-associated N
tion, those studies have been conducted almost
exclusively in boreal forests and subarctic tun-
dra, and attempts to extend this observation to