Simulation of stable carbon isotope dynamics of a C4 crop canopy

Simulation of the dynamics of ¹³CO₂ and discrimination processes of terrestrial ecosystems can provide important information on assessing regional carbon budgets. However, to date only a few process-based canopy models have been developed to simulate the stable carbon isotope dynamics, and fewer consider C4 plant canopies. In this study, a multi-layer biophysical canopy model was adapted to estimate the exchanges of CO₂ and energy between a maize (Zea mays L.) canopy and the atmosphere. In this model a photosynthesis module, based on an intercellular transport scheme of C4 photosynthetic pathway, was coupled to a Ball-Woodrow-Berry stomata conductance algorithm to simulate CO₂ assimilation and intercellular CO₂ concentration. Transport of CO₂ within and above the canopy was modeled using Lagrangian near field theory. Profiles of ¹²CO₂ and ¹³CO₂ concentrations, as well as photosynthetic discrimination within the canopy, were simulated. The modeled latent and sensible heat fluxes agreed well (&plusmn 20%) with eddy covariance measurements made during July 29 to August 5, 2003 at the Rosemount Research and Outreach Center of the University of Minnesota. The simulation of isotopic dynamics showed that on a diurnal basis, the photosynthetic discrimination ranged from 3.5 to 4.3 per mil. Shaded leaves showed slightly smaller discrimination than sun lit leaves due to the higher intercellular CO₂ concentration. Within the canopy, the minimum CO₂ concentration and the highest isotope ratio occurred at the level of about two thirds of the canopy height. These simulations provide detailed information on the distribution of carbon sources and sinks within the canopy. Further development and testing of the model is required in order to estimate carbon sources and sinks at regional scale.