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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-2018-505
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2018-505
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Dec 2018

Research article | 19 Dec 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Decadal impacts of nitrogen additions on temperate forest carbon sinks: A data-model comparison

Susan J. Cheng1, Peter G. Hess2, William R. Wieder3,4, R. Quinn Thomas5, Knute J. Nadelhoffer6, Julius Vira2, Danica L. Lombardozzi3, Per Gundersen7, Ivan J. Fernandez8, Patrick Schleppi9, Marie-Cécile Gruselle10, Filip Moldan11, and Christine L. Goodale1 Susan J. Cheng et al.
  • 1Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, USA
  • 2Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY, USA
  • 3National Center for Atmospheric Research, Boulder, CO, USA
  • 4Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, Colorado, USA
  • 5Department of Forest Resources and Environmental Conservation, Virginia Tech, Blacksburg, VA, USA
  • 6Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, Michigan, USA
  • 7Department of Geosciences and Natural Resource Management, University of Copenhagen, Denmark
  • 8Climate Change Institute and School of Forest Resources, University of Maine, Orono, ME, USA
  • 9Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland
  • 10Institute for Geography, University of Jena, Jena, Germany
  • 11IVL Swedish Environmental Research Institute, Box 53021, 40014, Gothenburg, Sweden

Abstract. To accurately capture the measured impacts of nitrogen (N) on the land carbon (C) sink in Earth system models, model responses to both N limitation and ecosystem N additions (e.g., from atmospheric N deposition and fertilizer) need to be evaluated. The response of the land C sink to N additions depends on the fate of these additions – that is, how much of the added N is lost from the ecosystem through N loss pathways, or recovered and used to increase C storage in plants and soils. Here, we evaluate the C-N dynamics of the latest version of a global land model, the Community Land Model 5 (CLM5). Because the default version of CLM5 overestimated the magnitude of N inputs and losses compared to observations, we configured an adjusted version of CLM5 with more conservative assumptions about these fluxes. We then compared the short- (< 3 years) and longer-term (5–17 years) simulations of N fate in CLM5 against observations from 13 long-term 15N tracer addition experiments at eight temperate forest sites. Both the default and adjusted configurations of CLM5 overestimated plant N recovery following N additions. In particular, the adjusted configuration simulated that plants acquired more than twice the amount of added N recovered in 15N tracer studies, on both short (CLM5: 46 % ± 12 %; observations: 18 % ± 12 %; mean across sites ±1 standard deviation) and longer timescales (CLM5: 23 % ± 6 %; observations: 13 % ± 5 %). The default version of CLM5 underestimated long-term 15N recovery in soils, while soil N recoveries in the adjusted configuration were closer to observations on both the short (CLM5: 40 % ± 10 %; observations: 54 % ± 22 %) and longer-term (CLM5: 59 % ± 15 %; observations: 69 % ± 18). However, in both configurations, soil N recoveries in CLM5 occurred from the cycling of N through plants rather than through direct immobilization in the soil, as often indicated by the tracer studies. Although CLM5 overestimated plant N recovery, the simulated increase in C stocks to recovered N was not larger than estimated by observations, largely because the model's assumed C : N ratio for wood was nearly half that suggested by field measurements at our sites. Overall, results suggest that simulating accurate ecosystem responses to changes in N additions requires increasing soil competition for N relative to plants, and examining model assumptions of C : N stoichiometry – which should also improve model estimates of other terrestrial C-N processes and interactions.

Susan J. Cheng et al.
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Status: final response (author comments only)
Status: final response (author comments only)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Susan J. Cheng et al.
Susan J. Cheng et al.
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Short summary
Nitrogen pollution and fertilizer can change how much carbon is stored in plant and soil stocks. Understanding how much added nitrogen is recovered in plants or soils is critical to estimating the size of the future land carbon sink. We compared how nitrogen additions are recovered in modeled soil and plant stocks against data from long-term nitrogen addition experiments. We found that the model simulates recovery of added nitrogen into soils through a different process than found in the field.
Nitrogen pollution and fertilizer can change how much carbon is stored in plant and soil stocks....
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