Global climate computer models like the Community Earth System Model (CESM) bring together a range of physical, chemical, and biological processes to create a complex portrait of the changing global climate. One portion of CESM is the Community Land Model, which represents the climate impact of terrestrial ecosystems. Plants play an important role in mediating carbon exchange between the land and the atmosphere by fixing atmospheric carbon dioxide (CO2) during photosynthesis. Nitrogen is a limiting factor that plants rely upon to get the CO2 they need. Nitrogen taken up through the plant’s roots is a key factor regulating the chlorophyll and Rubisco enzyme required for photosynthesis and mitochondrial enzymes that power the plant with adenosine triphosphate (ATP) energy and regulate respiration.
In a recent study, Ghimire et al. question whether nitrogen’s role in regulating carbon exchange is properly accounted for in the most recent version of the Community Land Model (CLM4.5). The team found that the model underestimates how much CO2 plants fix each day in scenarios where nitrogen is a limiting factor, an error that nudges the model’s intricate portrayal of interconnected systems away from reality.
The authors reasoned that the problem rests with the current model’s method for calculating the relationship between a plant’s nitrogen uptake and carbon fixation. CLM4.5 uses instantaneous downregulation to account for limitations associated with nitrogen availability. In this method, nitrogen limitations are initially ignored in photosynthesis calculations, and nitrogen availability is accounted for by downregulating the calculated potential photosynthetic rate. In their new approach, the team used the predicted amount of nitrogen comprising the Rubisco enzyme in plant leaves to explicitly constrain photosynthesis. This approach allowed them to use leaf trait observational data to constrain the model.
The authors found that CLM4.5 does not represent nitrogen’s role as a limiting factor in photosynthesis in another important way; that is, it links root nitrogen uptake to processes that occur in the leaves rather than in the roots. Since a range of chemical and physical factors influences how efficiently roots access soil nutrients, the team accounted for these root-related processes in calculating how much nitrogen plant leaves have to work with.
The team found that by altering the existing CLM4.5 model to more accurately represent nitrogen’s relationship to photosynthesis, they could more accurately represent carbon fixation in photosynthesis, leaf area, and biomass. By providing an improved, more comprehensive understanding of terrestrial carbon cycling, the model serves as a more useful tool for understanding global climate. (Journal of Advances in Modeling Earth Systems (JAMES), doi:10.1002/2015MS000538, 2016)
—Shannon Kelleher, Writer Intern
Citation: Kelleher, S. (2016), Nitrogen garners starring role in refined Earth system model, Eos, 97, doi:10.1029/2016EO050603. Published on 19 April 2016.