Elimination of Intraspecific Competition Does Not Improve Maize Leaf Physiological and Biochemical Responses to Topsoil Degradation

Soil degradation limits maize grain yield, but the mechanisms by which leaf functions respond to topsoil depth and their contributions to yield are unclear. We quantified the response mechanisms of leaf functions to topsoil depth with topsoil depths of 10 cm (S₁), 20 cm (S₂), 30 cm (S₃), 40 cm (S₄), and 50 cm (S₅) and planting densities of 15,000 plants ha^-1 (D₁, the plant spacing was 111.1 cm and there was no mutual influence between individuals) and 75,000 plants ha^-1 (D₂). The grain yield in S₁ was significantly lower than that in S₂, S₃, S₄, and S₅, and the maximum reductions in yield were 39.7% in D₁ and 39.1% in D₂. The coefficients of variation for yield in S₁ and S₂ were significantly higher than those in S₃, S₄, and S₅ at both densities and in both years. The net assimilation rate and production efficiency of leaf area, as well as leaf nitrogen and carbon accumulation, all decreased with decreasing topsoil depth. The decreasing topsoil depth significantly reduced the maize leaf net photosynthetic rate, activities of key nitrogen metabolism enzymes, and photosynthesis. Therefore, eliminating intraspecific competition did not reduce the yield loss caused by a reduction in topsoil because leaf nitrogen metabolism and photosynthetic processes were severely limited by the decrease in topsoil depth.