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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-6
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-6
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 25 Mar 2020

Submitted as: research article | 25 Mar 2020

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This preprint is currently under review for the journal BG.

Century-scale wood nitrogen isotope trajectories from an oak savanna with variable fire frequencies

Matthew Trumper1, Daniel Griffin1, Sarah E. Hobbie2, Ian Howard3, David M. Nelson4, Peter B. Reich5,6, and Kendra K. McLauchlan7 Matthew Trumper et al.
  • 1Department of Geography, Society, and Environment, University of Minnesota, Minneapolis, MN 55455, USA
  • 2Department of Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, MN 55108, USA
  • 3Department of Geosciences, University of Arkansas, Fayetteville, AR 72701, USA
  • 4Appalachian Laboratory, University of Maryland Center for Environmental Science, Frostburg, MD 21532, USA
  • 5Department of Forest Resources, University of Minnesota, St Paul, MN 55108, USA
  • 6Hawkesbury Institute for the Environment, Western Sydney University, Penrith, New South Wales 2753, Australia
  • 7Department of Geography and Geospatial Sciences, Kansas State University, Manhattan, KS 66506, USA

Abstract. Fire frequency exerts a fundamental control on productivity and nutrient cycling in savanna ecosystems. A single fire event often increases short-term nitrogen (N) availability to individual plants, but repeated burning causes ecosystem carbon and N losses and can ultimately decrease soil organic matter and N availability. However, these effects remain poorly understood due to limited long-term biogeochemical data. Here, we leveraged one of the longest running prescribed burn experiments (established in 1964) to evaluate how fire frequency and changing vegetation composition influenced wood stable N isotopes (δ15N) across space and time. We developed multiple δ15N records across a burn frequency gradient from precisely dated Quercus macrocarpa tree-rings in an oak savanna at Cedar Creek Ecosystem Science Reserve, Minnesota, USA. Sixteen trees were sampled across four treatment stands that varied in temporal onset of burning and burn frequency, but were consistent in overstory species representation, soil characteristics, and topography. Burn frequency ranged from an unburned control stand to a high fire-frequency stand that burned in four of every five years during the past 55 years. Because N stocks and net N mineralization rates are currently lowest in frequently burned stands, we hypothesized that wood δ15N trajectories would have declined over time in all burned stands, but at a rate proportional to fire frequency. We found that wood δ15N records within each stand were remarkably coherent in their mean state and trend through time. A gradual, temporally synchronous decline in wood δ15N occurred in the mid 20th century in the no-, low-, and medium-fire stands, whereas there was no trend in the high-fire stand. The decline in the three stands did not systematically coincide with the onset of prescribed burning. Thus, we found limited evidence for variation in wood δ15N that could be attributed directly to long-term fire frequency in this prescribed burn experiment in temperate oak savanna. Our wood δ15N results may instead reflect decadal-scale changes in vegetation composition and abundance due to early to mid 20th century fire suppression.

Matthew Trumper et al.

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Matthew Trumper et al.

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Latest update: 04 Apr 2020
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Short summary
We developed century-scale records of wood nitrogen isotopes (δ15N) from 16 trees across a long-term savanna fire experiment. Results show similar long-term trajectories in three out of four burn treatments. Lack of evidence to support our hypotheses underscores the complexity of nitrogen dynamics inferred from wood δ15N. This is the first study to our knowledge to investigate multi-decadal effects of fire at different return intervals on wood δ15N, a potential proxy of nitrogen availability.
We developed century-scale records of wood nitrogen isotopes (δ15N) from 16 trees across a...
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