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Abstract:
Terrestrial carbon (C) cycle models applied for climate projections simulate a strong increase in net primary
productivity (NPP) due to elevated atmospheric CO2 concentration
during the 21st century. These models usually neglect
the limited availability of nitrogen (N) and phosphorus (P),
nutrients that commonly limit plant growth and soil carbon
turnover. To investigate how the projected C sequestration
is altered when stoichiometric constraints on C cycling are
considered, we incorporated a P cycle into the land surface
model JSBACH (Jena Scheme for Biosphere–Atmosphere
Coupling in Hamburg), which already includes representations
of coupled C and N cycles.
The model reveals a distinct geographic pattern of P and
N limitation. Under the SRES (Special Report on Emissions
Scenarios) A1B scenario, the accumulated land C uptake between
1860 and 2100 is 13% (particularly at high latitudes)
and 16% (particularly at low latitudes) lower in simulations
with N and P cycling, respectively, than in simulations without
nutrient cycles. The combined effect of both nutrients reduces
land C uptake by 25% compared to simulations without
N or P cycling. Nutrient limitation in general may be
biased by the model simplicity, but the ranking of limitations
is robust against the parameterization and the inflexibility
of stoichiometry. After 2100, increased temperature and high CO2 concentration cause a shift from N to P limitation
at high latitudes, while nutrient limitation in the tropics
declines. The increase in P limitation at high-latitudes is induced
by a strong increase in NPP and the low P sorption
capacity of soils, while a decline in tropical NPP due to high
autotrophic respiration rates alleviates N and P limitations.
The quantification of P limitation remains challenging. The
poorly constrained processes of soil P sorption and biochemical
mineralization are identified as the main uncertainties in
the strength of P limitation. Even so, our findings indicate
that global land C uptake in the 21st century is likely overestimated
in models that neglect P and N limitations. In the
long term, insufficient P availability might become an important
constraint on C cycling at high latitudes. Accordingly,
we argue that the P cycle must be included in global models used for C cycle projections.