Abstract. 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 that would 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 were closer to observations in the short term
(CLM5: 40±10 %; observations: 54±22 %) but smaller
than observations in the long 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 open and 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 stocks to 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 at the
field 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.
Biomass increases attributed to both faster tree growth and altered allometric relationships under long‐term carbon dioxide enrichment at a temperate forest