Carbon-Nitrogen Interactions and Feedbacks Between Climate and the Terrestrial Carbon Cycle.

Carbon uptake by terrestrial ecosystems plays an important role in defining changes in the atmospheric CO₂ concentration and changes in climate.  In turn, carbon uptake is influenced by these changes.  It has long been recognized that nitrogen limitations often constrain carbon accumulations in mid- and high-latitude ecosystems, such as temperate and boreal forests.  Recent research on plant responses to elevated CO₂ concentrations is also consistent with the idea that low nitrogen-availability can constrain carbon sequestration in terrestrial ecosystems.

Potential impacts of increased concentrations of atmospheric CO₂ on terrestrial ecosystems have been the subject of numerous modeling studies.  Most of the terrestrial biosphere models currently used in climate-change assessments do not consider nitrogen limitations on net carbon storage.  Thus, they probably exaggerate the terrestrial biosphere’s potential to accumulate carbon in response to CO₂ fertilization and so overestimate its potential to slow atmospheric CO₂ rise and the rate of climate change.

Here, we explore the consequences of considering carbon-nitrogen interactions on the feedbacks between climate and the terrestrial carbon cycle using the Terrestrial Biogeochemistry Model (TEM).  Considerations of carbon-nitrogen interactions not only limit the effects of CO₂ fertilization, but also change the sign of the feedback between the climate and the terrestrial carbon cycle.  In the absence of carbon-nitrogen interactions, surface warming significantly reduces sequestration in both vegetation and soil by increasing respiration and decomposition (a positive feedback).  However, if plant carbon uptake is assumed to be nitrogen limited, an increase in decomposition leads to an increase in nitrogen availability that enables plants to respond to increased concentrations of atmospheric CO₂.  The resulting increased carbon uptake by vegetation can exceed carbon loss from the soil, leading to increased carbon sequestration (a negative feedback).  Under very strong surface warming, however, terrestrial ecosystems become a carbon source whether or not carbon-nitrogen interactions are considered.