Limitation of Maize Potential Yield by Phosphorus at the Global Scale

Phosphorus (P) is known as a major limiting factor of crop yields at the global scale. Previous estimates of the global P limitation are either based on statistical approaches or on complex global gridded crop models. Both failed to distinguish between P and the other limiting factors. Global gridded crop models, despite their complexities, omitted key mechanisms such as soil P dynamics or plant adjustments to P limitation (e.g., change in root:shoot ratio or in shoot P concentration). Thus, current approaches fail to quantify the contribution of P limitation to the global yield gap. Here, we developed a simple but mechanistic model (called GPCROP) that simulates the interactions between plant growth and soil P at a daily time step, all other factors being assumed non-limiting. The model explicitly represents key mechanisms such as the replenishment of the soil P solution by more stable soil P pools, the diffusion of P in soil, and plant adjustments to P limitation. We found that soil available P greatly limits the global maize potential production, even when that limitation was strongly alleviated by plant adjustment mechanisms. With and without these adjustments, maize global production would decrease by 78.9% (std = 17.3) and 92.7% (std = 7.4), respectively, compared to its potential production. We also found that the beginning of the growing season is a key period for P limitation as roots, not yet developed, cannot sustain the plant P demand. This suggests that earlier studies based on a comparison between annual averages of soil supply versus plant demand are not appropriate for assessing P limitation. Considerable uncertainties remain in our approach, and we especially stress the need to use global datasets of soil iron and aluminum (hydr)oxides, currently in development, to constrain the spatial variation of some key parameters driving the P concentration of the soil solution.