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

Van Den Elzen, Eva; Kox, Martine A R; Harpenslager, Sarah F.; Hensgens, Geert LU ; Fritz, Christian; Jetten, Mike S M; Ettwig, Katharina F. and Lamers, Leon P M (2017) In Biogeosciences 14(5). p.1111-1122
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... (More)

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.

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Contribution to journal
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published
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Biogeosciences
volume
14
issue
5
pages
12 pages
publisher
Copernicus Publications
external identifiers
  • scopus:85015207610
  • wos:000396169300001
ISSN
1726-4170
DOI
10.5194/bg-14-1111-2017
language
English
LU publication?
yes
id
855470c7-f350-4e0f-ad3e-e736caedd9c1
date added to LUP
2017-03-29 12:43:35
date last changed
2017-09-18 11:34:25
@article{855470c7-f350-4e0f-ad3e-e736caedd9c1,
  abstract     = {<p>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.</p>},
  author       = {Van Den Elzen, Eva and Kox, Martine A R and Harpenslager, Sarah F. and Hensgens, Geert and Fritz, Christian and Jetten, Mike S M and Ettwig, Katharina F. and Lamers, Leon P M},
  issn         = {1726-4170},
  language     = {eng},
  month        = {03},
  number       = {5},
  pages        = {1111--1122},
  publisher    = {Copernicus Publications},
  series       = {Biogeosciences},
  title        = {Symbiosis revisited : Phosphorus and acid buffering stimulate N2 fixation but not Sphagnum growth},
  url          = {http://dx.doi.org/10.5194/bg-14-1111-2017},
  volume       = {14},
  year         = {2017},
}