Mechanisms behind the declining CO2 fertilization effects on plant growth and grain yield in soybean (Glycine max)

Although a global decline in the CO2 fertilization effect on vegetation productivity has recently been detected using Earth system models and satellite observations, the underlying ecological and physiological mechanisms remain poorly understood. In particular, the optimal atmospheric CO2 concentration for maximizing the CO2 fertilization effect is still unclear. In this study, we examined the optimal CO2 concentration for crop yield, plant growth, and leaf photosynthesis in soybean (Glycine max) plants utilizing environmental growth chambers to control CO2 concentration from 400 to 1,600 μmol mol-1 with intervals of 200 μmol mol-1. Our findings indicate that the optimal atmospheric CO2 concentrations for crop yield and plant growth are between 1,000 and 1,200 μmol mol-1. Similarly, the optimal CO2 concentration for leaf photosynthesis is approximately 1,200 μmol mol-1, at which point the CO2 fertilization effect reaches its maximum. Beyond this optimum, further increases in CO2 concentration not only reduce grain yield and plant biomass but also decrease leaf photosynthesis in soybeans. This demonstrates that high CO2 concentrations exceeding optimal levels have adverse effects on this critical grain crop. The physiological declines in the CO2 fertilization effect observed in soybean plants were modified by decreases in stomatal density and stomatal distribution regularity, biochemical and photochemical efficiency, and the expression level of photosynthetic genes at higher CO2 concentrations. Furthermore, this knowledge can contribute to a deeper understanding of the temporal dynamics of CO2 fertilization effects on terrestrial vegetation uptake and global carbon storage in the context of future climate change.