Consequences of Considering Carbon–Nitrogen Interactions on the Feedbacks between Climate and the Terrestrial Carbon Cycle

TLDR

Plant carbon uptake is limited by nitrogen, so increased decomposition raises nitrogen availability and stimulates plant growth. The study investigates how carbon–nitrogen dynamics affect the interaction between the terrestrial carbon cycle and climate using the MIT Integrated Global Systems Model. Simulations were performed with two versions of the IGSM Terrestrial Ecosystems Model, one including and one excluding carbon–nitrogen dynamics. Including carbon–nitrogen dynamics changes the climate–carbon cycle feedback from positive to negative for moderate warming, limits CO₂ fertilization, and leads to larger CO₂ increases in the model, implying that omitting these interactions underestimates the emission reductions needed for stabilization while reducing uncertainty through terrestrial–oceanic uptake compensation.

Abstract

Abstract The impact of carbon–nitrogen dynamics in terrestrial ecosystems on the interaction between the carbon cycle and climate is studied using an earth system model of intermediate complexity, the MIT Integrated Global Systems Model (IGSM). Numerical simulations were carried out with two versions of the IGSM’s Terrestrial Ecosystems Model, one with and one without carbon–nitrogen dynamics. Simulations show that consideration of carbon–nitrogen interactions not only limits the effect of CO2 fertilization but also changes the sign of the feedback between the climate and terrestrial carbon cycle. In the absence of carbon–nitrogen interactions, surface warming significantly reduces carbon sequestration in both vegetation and soil by increasing respiration and decomposition (a positive feedback). If plant carbon uptake, however, is assumed to be nitrogen limited, an increase in decomposition leads to an increase in nitrogen availability stimulating plant growth. The resulting increase in carbon uptake by vegetation exceeds carbon loss from the soil, leading to enhanced 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. Overall, for small or moderate increases in surface temperatures, consideration of carbon–nitrogen interactions result in a larger increase in atmospheric CO2 concentration in the simulations with prescribed carbon emissions. This suggests that models that ignore terrestrial carbon–nitrogen dynamics will underestimate reductions in carbon emissions required to achieve atmospheric CO2 stabilization at a given level. At the same time, compensation between climate-related changes in the terrestrial and oceanic carbon uptakes significantly reduces uncertainty in projected CO2 concentration.