Seasonal variation in nitrogen fixation and effects of climate change in a subarctic heath
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Background and aims
Nitrogen fixation associated with cryptogams is potentially very important in arctic and subarctic terrestrial ecosystems, as it is a source of new nitrogen (N) into these highly N limited systems. Moss-, lichen- and legume-associated N2 fixation was studied with high frequency (every second week) during spring, summer, autumn and early winter to uncover the seasonal variation in input of atmospheric N2 to a subarctic heath with an altered climate.
We estimated N2 fixation from ethylene production by acetylene reduction assay in situ in a field experiment with the treatments: long- vs. short-term summer warming using plastic tents and litter addition (simulating expansion of the birch forest).
N2 fixation activity was measured from late April to mid November and 33 % of all N2 was fixed outside the vascular plant growing season (Jun–Aug). This substantial amount underlines the importance of N2 fixation in the cold period. Warming increased N2 fixation two- to fivefold during late spring. However, long-term summer warming tended to decrease N2 fixation outside the treatment (tents present) period. Litter alone did not alter N2 fixation but in combination with warming N2 fixation increased, probably because N2 fixation became phosphorus limited under higher temperatures, which was alleviated by the P supply from the litter.
In subarctic heath, the current N2 fixation period extends far beyond the vascular plant growing season. Climate warming and indirect effects such as vegetation changes affect the process of N2 fixation in different directions and thereby complicate predictions of future N cycling.
KeywordsBryophytes Global change Lichens Litter addition Long- vs. short-term warming Nitrogen and phosphorus Plant cover
We thank Gosha Sylvester for laboratory assistance with chemical analyses at the University of Copenhagen, ANS for providing facilities for laboratory analyses and fieldwork, and Ida Vedel-Petersen for carrying out ARA in June and July 2011. We gratefully acknowledge financial support from “Docent, Dr. Scient. Lauritz W. Olsons rejsefond”, the Danish Council for Independent Research, and from the Danish National Research Foundation (CENPERM DNRF100). Also, we thank the anonymous reviewers and the subject editor for their highly valuable comments and suggestions.
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