Decadal fates and impacts of nitrogen additions on temperate forest carbon storage: a data-model comparison

Cheng Susan J. 1, Hess Peter G. 2, Wieder William R. 3,4, Thomas R. Quinn 5, Nadelhoffer Knute J. 6, Vira Julius 2, Lombardozzi Danica L. 3, Gundersen Per 7, Fernandez Ivan J. 8, Schleppi Patrick 9, Gruselle Marie-Cécile 10, Moldan Philippe 11, Goodale Christine L. 1

1 Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, USA
2 Department of Biological and Environmental Engineering, Cornell University, Ithaca, NY, USA
3 National Center for Atmospheric Research, Boulder, CO, USA
4 Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, Colorado, USA
5 Department of Forest Resources and Environmental Conservation, Virginia Tech, Blacksburg, VA, USA
6 Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, Michigan, USA
7 Department of Geosciences and Natural Resource Management, University of Copenhagen, Denmark
8 Climate Change Institute and School of Forest Resources, University of Maine, Orono, ME, USA
9 Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, Switzerland
10 Institute for Geography, University of Jena, Jena, Germany
11 Swedish Environmental Research Institute, Göteborg, Sweden

Biogeosci. 16 (2019): 2771-2793

DOI: 10.5194/bg-16-2771-2019


To accurately capture the 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 version 5 (CLM5), and how they vary when ecosystems have large N inputs and losses (i.e., an open N cycle) or small N inputs and losses (i.e., a closed N cycle). This comparison allows us to identify potential improvements to CLM5 thatwould apply to simulated N cycles along the open-to-closed spectrum. We also compare the short- (< 3 years) and longer-term (5–17 years) N fates in CLM5 against observations from 13 long-term 15N tracer addition experiments at eight temperate forest sites. Simulations using both open and closed N cycles overestimated plant N recovery following N additions. In particular, the model configuration with a closed N cycle simulated that plants acquired more than twice the amount of added N recovered in 15N tracer studies on short timescales (CLM5: 46±12%; observations: 18±12%; mean across sites±1 standard deviation) and almost twice as much on longer timescales (CLM5: 23±6%; observations: 13±5%). Soil N recoveries in simulations with closed N cycles werecloser to observations in the short term (CLM5: 40±10%; observations: 54±22%) but smaller than observations in thelong term (CLM5: 59±15%; observations: 69±18%). Simulations with open N cycles estimated similar patterns in plant and soil N recovery, except that soil N recovery was also smaller than observations in the short term. In both openand closed sets of simulations, soil N recoveries in CLM5 occurred from the cycling of N through plants rather than through direct immobilization in the soil, as is often indicated by tracer studies. Although CLM5 greatly overestimated plant N recovery, the simulated increase in C stocksto recovered N was not much larger than estimated by observations, largely because the model’s assumed C:N ratio for wood was nearly half that suggested by measurements atthe field sites. Overall, results suggest that simulating accurate ecosystem responses to changes in N additions requiresincreasing 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.