Incorporating nitrogen cycle leads to predictions of more atmospheric CO2.
New climate research that accounts for the nutrient requirements of vegetation and other aspects of the nitrogen cycle suggests that over the coming century ecosystems could accumulate 2-3 times less carbon than previously estimated. The likely consequence is that levels of atmospheric CO2 will be significantly higher than predicted, researchers conclude in a recent report in Biogeosciences. Previous climate models that focused on links between planetary carbon balances and climate have largely overlooked the nitrogen cycle, assuming that nitrogen is readily abundant as a plant nutrient worldwide. However, ecologists have long recognized that nitrogen is limited in some ecosystems, including temperate and boreal forests, which are considered critical carbon reservoirs. A previous study that assessed the impact of terrestrial carbon-nitrogen interactions on global carbon balances concluded that if models neglect this factor, they will substantially underestimate the reductions in carbon emissions required to stabilize atmospheric CO2. In the new study, led by Peter Thornton of Oak Ridge National Laboratory, the researchers modified a state-of-the-art climate model to reflect interactions between changing levels of carbon and nitrogen under different scenarios. Although the model incorporated some ocean data, the simulations primarily addressed two opposing climate responses in terrestrial ecosystems: the fertilization of plant growth by mounting levels of atmospheric CO2; and the release of carbon, driven by warming and shifts in precipitation patterns. Including nitrogen-cycle effects in the model produced more nuanced results than those from less complex models. Accounting for nitrogen limitation in some of the 17 types of plant communities included in the simulations lowered the estimated quantities of carbon