EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Reductions in photosynthetic nitrogen demand due to elevated CO2 increases simulated future ecosystem carbon storage

Nicholas Smith1, Qing Zhu2, Trevor Keenan2,3, and William Riley2
Nicholas Smith et al.
  • 1Texas Tech University, Lubbock, USA (
  • 2Lawrence Berkeley National Lab, Berkeley, USA
  • 3University of California Berkeley, Berkeley, USA

Photosynthesis is the largest flux of carbon between the atmosphere and Earth’s surface and is driven by proteins that require nitrogen. Thus, photosynthesis is a key linkage between the terrestrial carbon and nitrogen cycles, and the representation of this linkage is  critical for coupled carbon-nitrogen land surface models. Most models use a scheme that assumes that photosynthetic nitrogen is driven by soil nitrogen availability. This contributes to projected future reductions in the CO2 fertilization of photosynthesis, as this fertilization becomes limited by nitrogen availability. However, recent results suggest that photosynthetic nitrogen is determined by leaf nitrogen demand, which is set by aboveground conditions, and that future increases in temperature and atmospheric CO2 should reduce photosynthetic nitrogen demand. Here, we used recently developed photosynthetic optimality theory to incorporate the effect of reduced photosynthetic demand for nitrogen into the land surface component of the Energy Exascale Earth System Model (ELM). We simulated land surface processes under future elevated CO2 conditions to 2100 using the RCP 8.5 scenario. Our simulations showed that photosynthesis increases under future conditions, but leaf nitrogen declines. This nitrogen savings led to an increase in simulated leaf area, which increased GPP and ecosystem carbon in 2100. These results suggest that land surface models may overestimate future nitrogen limitation of photosynthesis if they do not incorporate future reductions in photosynthetic nitrogen demand.

How to cite: Smith, N., Zhu, Q., Keenan, T., and Riley, W.: Reductions in photosynthetic nitrogen demand due to elevated CO2 increases simulated future ecosystem carbon storage, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1847,, 2022.