Johnson, D.W.1998.
Effects of elevated CO2 on nitrogen cycling:
A theoretical analysis. Tree Physiology. (submitted)
The interactions of elevated CO2 and N cycling are explored based on
hypotheses developed using conceptual models "tested" using simulations
generated by the Nutrient Cycling Model (NuCM). Based on the conceptual
models, four hypotheses were posed: (1) N limitations will preclude growth
responses to CO2 unless N cycles are altered; (2) reduced foliar N with
elevated CO2 will cause increased biomass/N and temporary decreases in N
cycling growth; (3) reduced wood N concentration with elevated CO2 will
cause increased biomass/N and growth; and (4) increased soil N
mineralization will cause only temporary increases in N uptake and growth.
NuCM simulations
for Walker Branch, TN and Duke, NC supported hypothesis 1 and also indicated
that the allocation of N to litterfall and away from wood increment increases
N cycling but reduces growth. NuCM predicted no increase in biomass/N or
growth with reduced foliar N and thus did not support hypothesis 2. NuCM
predicted increased biomass/N and growth with reduced wood N concentration,
however, and thus supported hypothesis 3. However, NuCM predicted that
reduced wood N concentration would also result in lower N uptake. NuCM
supported hypothesis 4 for Walker Branch but not Duke: at Duke, NuCM predicted
a longterm competitive advantage for the trees in terms of N uptake and growth
if N mineralization increased. Thus, the NuCm simulations provided some
interesting and counterintuitive results which would not have been obvious
based upon the simpler conceptual models alone. Whether the precictions of the
simpler models or NuCM hold true will have to await the results of ecosystem-level CO2 experimental studies.