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The bacterial communities of Alaskan mosses and their contributions to N2-fixation
Background
Mosses in high-latitude ecosystems harbor diverse bacterial taxa, including N
2
-fixers which are key contributors to nitrogen dynamics in these systems. Yet the relative importance of moss host species, and environmental factors, in structuring these microbial communities and their N
2
-fixing potential remains unclear. We studied 26 boreal and tundra moss species across 24 sites in Alaska, USA, from 61 to 69° N. We used cultivation-independent approaches to characterize the variation in moss-associated bacterial communities as a function of host species identity and site characteristics. We also measured N
2
-fixation rates via
15
N
2
isotopic enrichment and identified potential N
2
-fixing bacteria using available literature and genomic information.
Results
Host species identity and host evolutionary history were both highly predictive of moss microbiome composition, highlighting strong phylogenetic coherence in these microbial communities. Although less important, light availability and temperature also influenced composition of the moss microbiome. Finally, we identified putative N
2
-fixing bacteria specific to some moss hosts, including potential N
2
-fixing bacteria outside well-studied cyanobacterial clades.
Conclusions
The strong effect of host identity on moss-associated bacterial communities demonstrates mosses’ utility for understanding plant-microbe interactions in non-leguminous systems. Our work also highlights the likely importance of novel bacterial taxa to N
2
-fixation in high-latitude ecosystems.
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Video Abstract
Journal Article
Historical dictionary of Leibniz's philosophy
\"Historical Dictionary of Leibniz's Philosophy, Second Edition contains a chronology, an introduction, and an extensive bibliography. The dictionary section has more than 500 cross-referenced entries on Leibniz's philosophy, written work, teachers, contemporaries, and philosophers influenced by him\"-- Provided by publisher.
BOOK
The relationship of C and N stable isotopes to high-latitude moss-associated N₂ fixation
Moss-associated N₂ fixation by epiphytic microbes is a key biogeochemical process in nutrient-limited high-latitude ecosystems. Abiotic drivers, such as temperature and moisture, and the identity of host mosses are critical sources of variation in N₂ fixation rates. An understanding of the potential interaction between these factors is essential for predicting N inputs as moss communities change with the climate. To further understand the drivers and results of N₂ fixation rate variation, we obtained natural abundance values of C and N isotopes and an associated rate of N₂ fixation with ¹⁵N₂ gas incubations in 34 moss species collected in three regions across Alaska, USA. We hypothesized that δ¹⁵N values would increase toward 0‰ with higher N₂ fixation to reflect the increasing contribution of fixed N₂ in moss biomass. Second, we hypothesized that δ¹³C and N₂ fixation would be positively related, as enriched δ¹³C signatures reflect abiotic conditions favorable to N₂ fixation. We expected that the magnitude of these relationships would vary among types of host mosses, reflecting differences in anatomy and habitat. We found little support for our first hypothesis, with only a modest positive relationship between N₂ fixation rates and δ¹⁵N in a structural equation model. We found a significant positive relationship between δ¹³C and N₂ fixation only in Hypnales, where the probability of N₂ fixation activity reached 95% when δ¹³C values exceeded - 30.4‰. We conclude that moisture and temperature interact strongly with host moss identity in determining the extent to which abiotic conditions impact associated N₂ fixation rates.
Journal Article
Host Identity as a Driver of Moss-Associated N₂ Fixation Rates in Alaska
Moss-associated N₂ fixation provides a substantial but heterogeneous input of new N to nutrientlimited ecosystems at high latitudes. In spite of the broad diversity of mosses found in boreal and Arctic ecosystems, the extent to which host moss identity drives variation in N₂ fixation rates remains largely undetermined. We used ¹⁵N₂ incubations to quantify the fixation rates associated with 34 moss species from 24 sites ranging from 60° to 68° N in Alaska, USA. Remarkably, all sampled moss genera fixed N₂, including well-studied feather and peat mosses and genera such as Tomentypnum, Dicranum, and Polytrichum. The total moss-associated N₂ fixation rates ranged from almost zero to 3.2 mg N m⁻² d⁻¹, with an average of 0.8 mg N m⁻² d⁻¹, based on abundance-weighted averages of all mosses summed for each site. Random forest models indicated that moss taxonomic family was a better predictor of rate variation across Alaska than any of the measured environmental factors, including site, pH, tree density, and mean annual precipitation and temperature. Consistent with this finding, mixed models showed that trends in N₂ fixation rates among moss genera were consistent across biomes. We also found “hotspots” of high fixation rates in one-fourth of sampled sites. Our results demonstrated the importance of moss identity in influencing N₂ fixation rates. This in turn indicates the potential utility of moss identity when making ecosystem N input predictions and exploring other sources of process rate variation.
Journal Article
Net nitrogen mineralization in Alberta bog peat is insensitive to experimentally increased nitrogen deposition and time since wildfire
Across northern Alberta, Canada, bogs experience periodic wildfire and, in the Fort McMurray region, are exposed to increasing atmospheric N deposition related to oil sands development. As the fire return interval shortens and/or growing season temperatures increase, the regional peatland CO₂–C sink across northern Alberta will likely decrease, but the magnitude of the decrease could be diminished if increasing atmospheric N deposition alters N cycling in a way that stimulates post-fire successional development in bogs. We quantified net ammonification, nitrification, and dissolved organic N (DON) production in surface peat along a post-fire chronosequence of five bogs where we also experimentally manipulated N deposition (no water controls plus 0, 10, and 20 kg N ha⁻¹ yr⁻¹ simulated deposition, as NH₄NO₃). Initial KCl-extractable NH₄⁺–N, NO₃⁻–N and DON averaged 176 ± 6, 54 ± 0.2, and 3580 ± 40 ng N cm⁻³, respectively, with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Net ammonification, nitrification, and DON production averaged 3.8 ± 0.3, 1.6 ± 0.2, and 14.3 ± 2.0 ng N cm⁻³ d⁻¹, also with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Our hypothesis that N mineralization would be stimulated after fire because root death would create a pulse of labile soil organic C was not supported, most likely because ericaceous plant roots typically are not killed in boreal bog wildfires. The absence of any N mineralization response to experimental N addition is most likely a result of rapid immobilization of added NH₄⁺–N and NO₃⁻–N in peat with a wide C:N ratio. In these boreal bogs, belowground N cycling is likely characterized by large DON pools that turn over relatively slowly and small DIN pools that turn over relatively rapidly. For Alberta bogs that have persisted at historically low N deposition values and begin to receive higher N deposition related to anthropogenic activities, peat N mineralization processes may be largely unaffected until the peat C:N ratio reaches a point that no longer favors immobilization of NH₄⁺–N and NO₃⁻–N.
Journal Article
The relationship of C and N stable isotopes to high-latitude moss-associated N.sub.2 fixation
Moss-associated N.sub.2 fixation by epiphytic microbes is a key biogeochemical process in nutrient-limited high-latitude ecosystems. Abiotic drivers, such as temperature and moisture, and the identity of host mosses are critical sources of variation in N.sub.2 fixation rates. An understanding of the potential interaction between these factors is essential for predicting N inputs as moss communities change with the climate. To further understand the drivers and results of N.sub.2 fixation rate variation, we obtained natural abundance values of C and N isotopes and an associated rate of N.sub.2 fixation with .sup.15N.sub.2 gas incubations in 34 moss species collected in three regions across Alaska, USA. We hypothesized that [delta].sup.15N values would increase toward 0â° with higher N.sub.2 fixation to reflect the increasing contribution of fixed N.sub.2 in moss biomass. Second, we hypothesized that [delta].sup.13C and N.sub.2 fixation would be positively related, as enriched [delta].sup.13C signatures reflect abiotic conditions favorable to N.sub.2 fixation. We expected that the magnitude of these relationships would vary among types of host mosses, reflecting differences in anatomy and habitat. We found little support for our first hypothesis, with only a modest positive relationship between N.sub.2 fixation rates and [delta].sup.15N in a structural equation model. We found a significant positive relationship between [delta].sup.13C and N.sub.2 fixation only in Hypnales, where the probability of N.sub.2 fixation activity reached 95% when [delta].sup.13C values exceeded - 30.4â°. We conclude that moisture and temperature interact strongly with host moss identity in determining the extent to which abiotic conditions impact associated N.sub.2 fixation rates.
Journal Article
Host Identity as a Driver of Moss-Associated N.sub.2 Fixation Rates in Alaska
Moss-associated N.sub.2 fixation provides a substantial but heterogeneous input of new N to nutrient-limited ecosystems at high latitudes. In spite of the broad diversity of mosses found in boreal and Arctic ecosystems, the extent to which host moss identity drives variation in N.sub.2 fixation rates remains largely undetermined. We used .sup.15N.sub.2 incubations to quantify the fixation rates associated with 34 moss species from 24 sites ranging from 60° to 68° N in Alaska, USA. Remarkably, all sampled moss genera fixed N.sub.2, including well-studied feather and peat mosses and genera such as Tomentypnum, Dicranum, and Polytrichum. The total moss-associated N.sub.2 fixation rates ranged from almost zero to 3.2 mg N m.sup.-2 d.sup.-1, with an average of 0.8 mg N m.sup.-2 d.sup.-1, based on abundance-weighted averages of all mosses summed for each site. Random forest models indicated that moss taxonomic family was a better predictor of rate variation across Alaska than any of the measured environmental factors, including site, pH, tree density, and mean annual precipitation and temperature. Consistent with this finding, mixed models showed that trends in N.sub.2 fixation rates among moss genera were consistent across biomes. We also found \"hotspots\" of high fixation rates in one-fourth of sampled sites. Our results demonstrated the importance of moss identity in influencing N.sub.2 fixation rates. This in turn indicates the potential utility of moss identity when making ecosystem N input predictions and exploring other sources of process rate variation.
Journal Article
The Role of Host Identity in High Latitude Moss-Associated Nitrogen Fixation
Mosses make up a significant portion of primary plant productivity in Arctic and boreal ecosystems and are important regulators of biogeochemical cycling. In addition to producing recalcitrant litter and insulating soils, mosses often host epiphytic microbes capable of fixing nitrogen (N) from the air at rates which make it the largest source of a limiting nutrient in these environments. Since the availability of N is linked to carbon (C) fixation and decomposition, the current and future rates of N2 fixation are important topics of research in an area which stores large amounts of C belowground. Past evidence indicates that host moss identity and environmental conditions can alter rates of moss-associated N2 fixation. However, past studies often focus on a limited number of species and use indirect methods to measure N2 fixation. This dissertation employs 15N2 incubations to measure rates of moss-associated fixation at sites ranging from 60° to 68° N in Alaska in both natural surveys and manipulative experiments in the field. We found that N2 fixation is almost ubiquitous among mosses and that moss identity is consistently an important predictor of associated N2 fixation rates. In subsequent analyses related to C stable isotopes and a reciprocal transplant, we also found a significant interaction between host identity and environment. The strength of the interaction term was typically host specific. As temperature and other abiotic conditions change along with climate and cause changes in moss biomass and diversity, it is critical to incorporate the interaction term into predictions of future N inputs.
Dissertation
The bacterial communities of Alaskan mosses and their contributions to N 2 -fixation
Mosses in high-latitude ecosystems harbor diverse bacterial taxa, including N
-fixers which are key contributors to nitrogen dynamics in these systems. Yet the relative importance of moss host species, and environmental factors, in structuring these microbial communities and their N
-fixing potential remains unclear. We studied 26 boreal and tundra moss species across 24 sites in Alaska, USA, from 61 to 69° N. We used cultivation-independent approaches to characterize the variation in moss-associated bacterial communities as a function of host species identity and site characteristics. We also measured N
-fixation rates via
N
isotopic enrichment and identified potential N
-fixing bacteria using available literature and genomic information.
Host species identity and host evolutionary history were both highly predictive of moss microbiome composition, highlighting strong phylogenetic coherence in these microbial communities. Although less important, light availability and temperature also influenced composition of the moss microbiome. Finally, we identified putative N
-fixing bacteria specific to some moss hosts, including potential N
-fixing bacteria outside well-studied cyanobacterial clades.
The strong effect of host identity on moss-associated bacterial communities demonstrates mosses' utility for understanding plant-microbe interactions in non-leguminous systems. Our work also highlights the likely importance of novel bacterial taxa to N
-fixation in high-latitude ecosystems. Video Abstract.
Journal Article