Increased topsoil depth required to support increased grain yield production in high density maize

Globally, degradation of topsoil has significantly reduced soil productivity, while simultaneously, increasing planting density of maize has increased competition for limited resources in that topsoil. The objectives of this study were to 1) determine the effect of topsoil depth on grain yield, 2) assess the contributions of root structure and function, leaf area index, and photosynthetic rate to topsoil depth-induced grain yield, and 3) characterize the differences in yield, yield components, and physiological characteristics in response to changes in topsoil depth under different densities of maize. The field experiment was a 2 × 5 (two plant densities and five topsoil depths) factorial design with 10 treatment combinations. Plant densities were 60,000 plants ha−1 (D1) and 90,000 plants ha−1 (D2), and topsoil depths were 10 cm (S1), 20 cm (S2), 30 cm (S3), 40 cm (S4), and 50 cm (S5). The study was conducted in 2021 and 2022. Increasing topsoil depths significantly increased maize grain yield, and over the two years. The lowest values were in S1 and the highest were in S5, and the maximum increases in yield with increases in topsoil depth were 44.1% in D1 and 55.2% in D2. According to regression analysis, maximum grain yield would be obtained at topsoil depths of 59.1 cm in D1 and 61.0 cm in D2. In addition, to achieve yield increases of more than 15%, 10%, and 5% (10-cm range), topsoil depths in D1 would be 35.5 cm, 38.9 cm, and 42.7 cm, respectively, whereas in D2, the equivalent depths would be 38.0 cm, 42.4 cm, and 46.2 cm, respectively. Increasing topsoil depths also increased root weight (WR), length (LR), surface area (SR), volume (VR), and leaf area index (LAI), net assimilation rate of leaf area, and photosynthetic rate (Pn) and decreased harvest index (HI), with WR, LR, SR, VR, LAI, and Pn closely related to yield increase. Increasing topsoil depth effectively increased and stabilized maize yields, and the benefits of increases in topsoil depth were greater in the high-density planting than in the low-density planting, with greater benefits closely related to physiological contributions induced by topsoil depth. The study provides a viable theoretical basis for tillage strategies to increase maize grain yields by quantifying relationship between maize grain yield and topsoil depth, which in turn can promote sustainable agricultural development.