Articles | Volume 14, issue 5
Biogeosciences, 14, 1111–1122, 2017
Biogeosciences, 14, 1111–1122, 2017

Research article 09 Mar 2017

Research article | 09 Mar 2017

Symbiosis revisited: phosphorus and acid buffering stimulate N2 fixation but not Sphagnum growth

Eva van den Elzen1, Martine A. R. Kox2, Sarah F. Harpenslager3, Geert Hensgens4, Christian Fritz1,5,6, Mike S. M. Jetten2, Katharina F. Ettwig2, and Leon P. M. Lamers1,7 Eva van den Elzen et al.
  • 1Department of Aquatic Ecology & Environmental Biology, Institute for Water and Wetland Research, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, the Netherlands
  • 2Department of Microbiology, Institute for Water and Wetland Research, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, the Netherlands
  • 3School of Biological and Chemical Sciences, Queen Mary University, Mile End Road, E1 4NS, London, UK
  • 4Department of Physical Geography and Ecosystem Science, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
  • 5Centre for Energy and Environmental Studies, University of Groningen, Nijenborgh
  • 6Sustainable Agriculture, Rhein-Waal University for Applied Science, Wiesenstraße 5, 47575 Kleve, Germany
  • 7B-Ware Research Centre, Toernooiveld 1, 6525 ED Nijmegen, the Netherlands

Abstract. In pristine Sphagnum-dominated peatlands, (di)nitrogen (N2) fixing (diazotrophic) microbial communities associated with Sphagnum mosses contribute substantially to the total nitrogen input, increasing carbon sequestration. The rates of symbiotic nitrogen fixation reported for Sphagnum peatlands, are, however, highly variable, and experimental work on regulating factors that can mechanistically explain this variation is largely lacking. For two common fen species (Sphagnum palustre and S. squarrosum) from a high nitrogen deposition area (25 kg N ha−1 yr−1), we found that diazotrophic activity (as measured by 15 − 15N2 labeling) was still present at a rate of 40 nmol N gDW−1 h−1. This was surprising, given that nitrogen fixation is a costly process. We tested the effects of phosphorus availability and buffering capacity by bicarbonate-rich water, mimicking a field situation in fens with stronger groundwater or surface water influence, as potential regulators of nitrogen fixation rates and Sphagnum performance. We expected that the addition of phosphorus, being a limiting nutrient, would stimulate both diazotrophic activity and Sphagnum growth. We indeed found that nitrogen fixation rates were doubled. Plant performance, in contrast, did not increase. Raised bicarbonate levels also enhanced nitrogen fixation, but had a strong negative impact on Sphagnum performance. These results explain the higher nitrogen fixation rates reported for minerotrophic and more nutrient-rich peatlands. In addition, nitrogen fixation was found to strongly depend on light, with rates 10 times higher in light conditions suggesting high reliance on phototrophic organisms for carbon. The contrasting effects of phosphorus and bicarbonate on Sphagnum spp. and their diazotrophic communities reveal strong differences in the optimal niche for both partners with respect to conditions and resources. This suggests a trade-off for the symbiosis of nitrogen fixing microorganisms with their Sphagnum hosts, in which a sheltered environment apparently outweighs the less favorable environmental conditions. We conclude that microbial activity is still nitrogen limited under eutrophic conditions because dissolved nitrogen is being monopolized by Sphagnum. Moreover, the fact that diazotrophic activity can significantly be upregulated by increased phosphorus addition and acid buffering, while Sphagnum spp. do not benefit, reveals remarkable differences in optimal conditions for both symbiotic partners and calls into question the regulation of nitrogen fixation by Sphagnum under these eutrophic conditions. The high nitrogen fixation rates result in high additional nitrogen loading of 6 kg ha−1 yr−1 on top of the high nitrogen deposition in these ecosystems.

Short summary
Peatlands are important because they sequester large amounts of carbon, for which nitrogen is needed. In peatlands dominated by peat mosses, atmospheric nitrogen is fixed by associated microorganisms. We here show for the first time experimentally that phosphorus availability and acid buffering, both showing large variations among peatlands, can explain the strong differences reported for nitrogen fixation. This improves our understanding of peatland functioning in relation to global change.
Final-revised paper