Toward an impact assessment of ozone on plant carbon fixation using a process-based plant growth model: A case study of Fagus crenata grown under different soil nutrient levels.

Ozone (O) in the troposphere, an air pollutant with phytotoxicity, is considered as a driver of global warming, because it reduces plant carbon fixation. Recently, a process-based plant growth model has been used in evaluating the O impacts on plants (Schauberger et al., 2019). To make the evaluation more rigorous, we developed a plant growth model and clarified the key factors driving O-induced change in the whole-plant carbon fixation amount (C). Fagus crenata seedlings were exposed to three O levels (charcoal-filtered air or 1.0- or 1.5-folds ambient [O]) with three soil fertilization levels (non-, low-, or high-fertilized), i.e., a total of nine treatments. The C was reduced in non- and low-fertilized treatments but was unaffected in high-fertilized treatment by O fumigation. Our plant growth model could simulate C accurately (<10% error) by considering the impacts of O on plant leaf area and photosynthetic capacities, including maximum velocities of carboxylation and electron transport (V and J, respectively), and the initial slope and convexity of the curve of the electron transport velocity response to photosynthetic photon flux density (φ and θ, respectively). Furthermore, the model revealed that changes in V and J, φ and θ, or leaf area, caused by 1.5-folds the ambient [O] fumigation resulted in the following C changes: -1.6, -5.8, or -16.4% in non-fertilized seedlings, -4.1, -4.4, or -9.3% in low-fertilized seedlings, and -4.6, -7.6, or +5.8% in high-fertilized seedlings. Therefore, photosynthetic capacities (particularly φ and θ) and leaf area are important factors influencing the impact of O on C of F. crenata seedlings grown under various fertilization levels. Further, the impacts of O and soil nutrient on these photosynthetic capacities and plant leaf area should be considered to predict O-induced changes in carbon fixation by forest tree species using the process-based plant growth model.