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.
Control of soil N cycle processes byPteridium aquilinumandErica cinereain heathlands along a pH gradient
