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- Title
Decadal impacts of nitrogen additions on temperate forest carbon sinks: A data-model comparison.
- Authors
Cheng, Susan J.; Hess, Peter G.; Wieder, William R.; Thomas, R. Quinn; Nadelhoffer, Knute J.; Vira, Julius; Lombardozzi, Danica L.; Gundersen, Per; Fernandez, Ivan J.; Schleppi, Patrick; Gruselle, Marie-Cécile; Moldan, Filip; Goodale, Christine L.
- 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.
- Subjects
TEMPERATE forests; CARBON cycle; SEDIMENTATION &; deposition; BIOGEOCHEMISTRY; EARTH system science; ATMOSPHERIC nitrogen dioxide
- Publication
Biogeosciences Discussions, 2018, p1
- ISSN
1810-6277
- Publication type
Article
- DOI
10.5194/bg-2018-505