Incorporating nitrogen cycle leads to predictions of more atmospheric CO2

Noreen Parks in Environmental Science & Technology: 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.

….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. … The researchers found good agreement between their model’s representation of the nitrogen cycle and experimental data from a study on nitrogen and plant nutrition carried out at the Harvard Forest.

…Chris Field of the Carnegie Institution for Science says that “without biogeochemical feedbacks—especially carbon−nitrogen interactions—we know the models can’t be right, and this paper makes an impressive effort at addressing that deficit. While it’s difficult to assess the quantitative accuracy of the findings at this point, they make sense, and the conclusion that CO2 fertilization is constrained by nitrogen fits with a wide range of evidence that the fertilization effect is not likely to produce a huge increase [in] carbon storage.”…

Schematic illustrating feedback pathways included in the researchers’ model. Blue arrows designate processes represented in previous carbon-only land model components; orange arrows show the additional processes represented in the coupled carbon−nitrogen land model.
PETER THORNTON, ORNL