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Abstract Peat mosses (Sphagnumspp.) are keystone species in boreal peatlands, where they dominate net primary productivity and facilitate the accumulation of carbon in thick peat deposits.Sphagnummosses harbor a diverse assemblage of microbial partners, including N2‐fixing (diazotrophic) and CH4‐oxidizing (methanotrophic) taxa that support ecosystem function by regulating transformations of carbon and nitrogen. Here, we investigate the response of theSphagnumphytobiome (plant + constituent microbiome + environment) to a gradient of experimental warming (+0°C to +9°C) and elevated CO2(+500 ppm) in an ombrotrophic peatland in northern Minnesota (USA). By tracking changes in carbon (CH4, CO2) and nitrogen (NH4‐N) cycling from the belowground environment up toSphagnumand its associated microbiome, we identified a series of cascading impacts to theSphagnumphytobiome triggered by warming and elevated CO2. Under ambient CO2, warming increased plant‐available NH4‐N in surface peat, excess N accumulated inSphagnumtissue, and N2fixation activity decreased. Elevated CO2offset the effects of warming, disrupting the accumulation of N in peat andSphagnumtissue. Methane concentrations in porewater increased with warming irrespective of CO2treatment, resulting in a ~10× rise in methanotrophic activity withinSphagnumfrom the +9°C enclosures. Warming's divergent impacts on diazotrophy and methanotrophy caused these processes to become decoupled at warmer temperatures, as evidenced by declining rates of methane‐induced N2fixation and significant losses of keystone microbial taxa. In addition to changes in theSphagnummicrobiome, we observed ~94% mortality ofSphagnumbetween the +0°C and +9°C treatments, possibly due to the interactive effects of warming on N‐availability and competition from vascular plant species. Collectively, these results highlight the vulnerability of theSphagnumphytobiome to rising temperatures and atmospheric CO2concentrations, with significant implications for carbon and nitrogen cycling in boreal peatlands.
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Widespread nitrogen fixation in sediments from diverse deep‐sea sites of elevated carbon loading
Summary Nitrogen fixation, the biological conversion of N2to NH3, is critical to alleviating nitrogen limitation in many marine ecosystems. To date, few measurements exist of N2fixation in deep‐sea sediments. Here, we conducted > 400 bottle incubations with sediments from methane seeps, whale falls and background sites off the western coast of the United States from 600 to 2893 m water depth to investigate the potential rates, spatial distribution and biological mediators of benthic N2fixation. We found that N2fixation was widespread, yet heterogeneously distributed with sediment depth at all sites. In some locations, rates exceeded previous measurements by > 10×, and provided up to 30% of the community anabolic growth requirement for nitrogen. Diazotrophic activity appeared to be inhibited by pore water ammonium: N2fixation was only observed if incubation ammonium concentrations were ≤ 25 μM, and experimental additions of ammonium reduced diazotrophy. In seep sediments, N2fixation was dependent on CH4and coincident with sulphate reduction, consistent with previous work showing diazotrophy by microorganisms mediating sulphate‐coupled methane oxidation. However, the pattern of diazotrophy was different in whale‐fall and associated reference sediments, where it was largely unaffected by CH4, suggesting catabolically different diazotrophs at these sites.
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- Award ID(s):
- 1634297
- PAR ID:
- 10065841
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Environmental Microbiology
- Volume:
- 20
- Issue:
- 12
- ISSN:
- 1462-2912
- Format(s):
- Medium: X Size: p. 4281-4296
- Size(s):
- p. 4281-4296
- Sponsoring Org:
- National Science Foundation
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