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      Moss-cyanobacteria associations as biogenic sources of nitrogen in boreal forest ecosystems


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          The biological fixation of atmospheric nitrogen (N) is a major pathway for available N entering ecosystems. In N-limited boreal forests, a significant amount of N 2 is fixed by cyanobacteria living in association with mosses, contributing up to 50% to the total N input. In this review, we synthesize reports on the drivers of N 2 fixation in feather moss-cyanobacteria associations to gain a deeper understanding of their role for ecosystem-N-cycling. Nitrogen fixation in moss-cyanobacteria associations is inhibited by N inputs and therefore, significant fixation occurs only in low N-deposition areas. While it has been shown that artificial N additions in the laboratory as well as in the field inhibit N 2 fixation in moss-cyanobacteria associations, the type, as well as the amounts of N that enters the system, affect N 2 fixation differently. Another major driver of N 2 fixation is the moisture status of the cyanobacteria-hosting moss, wherein moist conditions promote N 2 fixation. Mosses experience large fluctuations in their hydrological status, undergoing significant natural drying and rewetting cycles over the course of only a few hours, especially in summer, which likely compromises the N input to the system via N 2 fixation. Perhaps the most central question, however, that remains unanswered is the fate of the fixed N 2 in mosses. The cyanobacteria are likely to leak N, but whether this N is transferred to the soil and if so, at which rates and timescales, is unknown. Despite our increasing understanding of the drivers of N 2 fixation, the role moss-cyanobacteria associations play in ecosystem-N-cycling remains unresolved. Further, the relationship mosses and cyanobacteria share is unknown to date and warrants further investigation.

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              A unifying framework for dinitrogen fixation in the terrestrial biosphere.

              Dinitrogen (N(2)) fixation is widely recognized as an important process in controlling ecosystem responses to global environmental change, both today and in the past; however, significant discrepancies exist between theory and observations of patterns of N(2) fixation across major sectors of the land biosphere. A question remains as to why symbiotic N(2)-fixing plants are more abundant in vast areas of the tropics than in many of the mature forests that seem to be nitrogen-limited in the temperate and boreal zones. Here we present a unifying framework for terrestrial N(2) fixation that can explain the geographic occurrence of N(2) fixers across diverse biomes and at the global scale. By examining trade-offs inherent in plant carbon, nitrogen and phosphorus capture, we find a clear advantage to symbiotic N(2) fixers in phosphorus-limited tropical savannas and lowland tropical forests. The ability of N(2) fixers to invest nitrogen into phosphorus acquisition seems vital to sustained N(2) fixation in phosphorus-limited tropical ecosystems. In contrast, modern-day temperatures seem to constrain N(2) fixation rates and N(2)-fixing species from mature forests in the high latitudes. We propose that an analysis that couples biogeochemical cycling and biophysical mechanisms is sufficient to explain the principal geographical patterns of symbiotic N(2) fixation on land, thus providing a basis for predicting the response of nutrient-limited ecosystems to climate change and increasing atmospheric CO(2).

                Author and article information

                Front Microbiol
                Front Microbiol
                Front. Microbiol.
                Frontiers in Microbiology
                Frontiers Media S.A.
                17 June 2013
                : 4
                [1] 1School of Environment, Natural Resources and Geography, Bangor University Bangor, Gwynedd, UK
                [2] 2School of Environment and Forest Sciences, University of Washington Seattle, WA, USA
                Author notes

                Edited by: Per Bengtson, Lund University, Sweden

                Reviewed by: Marja A. Tiirola, University of Jyväskylä, Finland; Richard S. Winder, Natural Resources Canada, Canada

                *Correspondence: Kathrin Rousk, School of Environment, Natural Resources and Geography, Bangor University, Deiniol Road, Bangor, Gwynedd LL 57 2UW, UK e-mail: kathrin.rousk@ 123456gmx.net

                This article was submitted to Frontiers in Terrestrial Microbiology, a specialty of Frontiers in Microbiology.

                Copyright © 2013 Rousk, Jones and DeLuca.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.

                : 12 April 2013
                : 28 May 2013
                Page count
                Figures: 3, Tables: 1, Equations: 0, References: 119, Pages: 10, Words: 8680
                Review Article

                Microbiology & Virology
                acetylene reduction,boreal biome,bryophytes,global change,n-cycle,nitrogenase,symbioses


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