A number of observational studies indicate that carbon sequestration by terrestrial ecosystems in a world with an atmosphere richer in carbon dioxide and a warmer climate depends on the interactions between the carbon and nitrogen cycles. However, most terrestrial ecosystem models being used in climate-change assessments do not take into account these interactions. Here we explore how carbon/nitrogen interactions in terrestrial ecosystems affect feedbacks to the climate system using the MIT Integrated Global Systems Model (IGSM) and its terrestrial ecosystems submodel, the Terrestrial Ecosystems Model (TEM). We use two versions of TEM, one with (standard TEM) and one without (carbon-only TEM) carbon/nitrogen interactions. Feedbacks between climate and the terrestrial carbon cycle are estimated by comparing model response to an increase in atmospheric CO2 concentration with and without climate change. Overall, for small or moderate increases in surface temperatures, the terrestrial biosphere simulated by the standard TEM takes up less atmospheric carbon than the carbon-only version, resulting in a larger increase in atmospheric CO2 concentration for a given amount of carbon emitted. With strong surface warming, the terrestrial biosphere simulated by the standard TEM may still become a carbon source early in the 23rd century.

Our simulations also show that consideration of carbon/nitrogen interactions not only limits the effect of CO2 fertilization in the absence of climate change, but also changes the sign of the carbon feedback with climate change. In the simulations with the carbon-only version of TEM, surface warming significantly reduces carbon sequestration in both vegetation and soil, leading to a positive carbon-cycle feedback to the climate system. However, in simulations with standard TEM, the increased decomposition of soil organic matter with higher temperatures releases soil nitrogen to stimulate plant growth and carbon storage in the vegetation that is greater than the carbon lost from soil. As a result, sequestration of carbon in terrestrial ecosystems increases, in comparison to the fixed climate case, and the carbon cycle feedback to the climate system becomes negative for much of the next three centuries.