Mechanisms of topsoil depth drive differences in maize yield and photosynthetic carbon assimilation

• Increasing topsoil depth can increase maize grain yields. • 13 C labeling determined photosynthate accumulation under different topsoil depths. • Suitable topsoil depth can achieve high and stable maize yields. 1 Sustainable increase in maize yield is severely constrained by the continuing reduction in topsoil depth due to irrational farming practices and the effects of climate change. However, the mechanisms by which topsoil depth affects crop physiology and biochemistry remain unclear, particularly with respect to photosynthesis and carbon assimilation. To investigate the effects of topsoil depth on maize photosynthetic processes, carbon assimilation, and yield in the field, we used a two-factor random block design with five topsoil depths of 10 cm (S1), 20 cm (S2), 30 cm (S3), 40 cm (S4), and 50 cm (S5) at two planting densities of 60,000 plants ha −1 (conventional density, D1) and 90,000 plants ha −1 (high density, D2). Increasing topsoil depth significantly increased maize grain yield, with maximum increases of 61.7% in D1 and 72.1% in D2. Increasing topsoil depth also increased chlorophyll content, maximum photochemical efficiency ( F v / F m ), actual photochemical efficiency (ΦPSII), and photosynthetic enzyme activities, including ribulose-1,5-bisphosphate carboxylase (Rubisco), phosphoenolpyruvate carboxylase (PEPC), and pyruvate orthophosphate dikinase (PPDK). With the increases in those parameters, plants maintained the highest net photosynthetic rate ( P nmax ) when reaching the light saturation point, with maximum increases of 68.0% in D1 and 75.7% in D2, thereby increasing dry matter production at physiological maturity. The accumulation of 13 C-photosynthates in maize stem, leaf, and grain increased with the increase in topsoil depth, indicating increases in carbon assimilation capacity, distribution efficiency, and photosynthetic capacity. In summary, increasing topsoil depth is an important factor in ensuring high and stable maize yields, and the increase in yield is closely related to the physiological differences caused by changes in topsoil depth.