Abstract
Nitrogen cycle dynamics have the capacity to attenuate the magnitude of global
terrestrial carbon sinks and sources driven by CO2 fertilization and changes in climate. In
this study, two versions of the terrestrial carbon and nitrogen cycle components of the
Integrated Science Assessment Model (ISAM) are used to evaluate how variation in
nitrogen availability influences terrestrial carbon sinks and sources in response to changes
over the 20th century in global environmental factors including atmospheric CO2
concentration, nitrogen inputs, temperature, precipitation and land use. The two versions
of ISAM vary in their treatment of nitrogen availability: ISAM-NC has a terrestrial
carbon cycle model coupled to a fully dynamic nitrogen cycle while ISAM-C has an
identical carbon cycle model but nitrogen availability is always in sufficient supply.
Overall, the two versions of the model estimate approximately the same amount of global
mean carbon uptake over the 20th century. However, comparisons of results of ISAM-NC
relative to ISAM-C reveal that nitrogen dynamics: (1) reduced the 1990s carbon sink
associated with increasing atmospheric CO2 by 0.53 PgC yr1 (1 Pg = 1015g), (2) reduced
the 1990s carbon source associated with changes in temperature and precipitation of
0.34 PgC yr1 in the 1990s, (3) an enhanced sink associated with nitrogen inputs by
0.26 PgC yr1, and (4) enhanced the 1990s carbon source associated with changes in land
use by 0.08 PgC yr1 in the 1990s. These effects of nitrogen limitation influenced the
spatial distribution of the estimated exchange of CO2 with greater sink activity in
high latitudes associated with climate effects and a smaller sink of CO2 in the
southeastern United States caused by N limitation associated with both CO2 fertilization
and forest regrowth. These results indicate that the dynamics of nitrogen availability are
important to consider in assessing the spatial distribution and temporal dynamics of
terrestrial carbon sources and sinks.