Coordination of Density and Nitrogen Fertilization Improves Stalk Lodging Resistance of Strip-Intercropped Maize with Soybeans by Affecting Stalk Quality Traits

To ensure yield in strip-intercropped maize with soybeans (SM), it is crucial to plant at a density comparable to that of monoculture maize (MM). This requires reducing spacing by more than half, increasing intraspecific competition, and altering stalk lodging resistance traits compared with MM. Nitrogen fertilization can effectively mediate stalk lodging resistance. However, it is still unclear how nitrogen rates influence SM’s stalk lodging resistance under high-density conditions and how that resistance compares to MM. The experiment involved four N fertilizer treatments with two planting densities: medium density (60,000 plants/ha) and high density (75,000 plants/ha). Additionally, different planting patterns of strip-cropped (S) and monoculture (M) were implemented. The N fertilizer application rates were N0 (0 kg/ha), N225 (225 kg/ha), N300 (300 kg/ha), and N375 (375 kg/ha). The stalk lodging resistance was represented by the breaking strength of the third basal internode. The study revealed that, at the same planting density, the third basal internode of the stalk exhibited consistent results in terms of its diameter, crushing strength, total number and area of vascular bundles, and N content. Notably, all these traits exhibited a significant positive relationship with breaking strength. The highest values for these parameters and yield were observed under N225 and N300 fertilization rates for medium-density monoculture and strip-cropped maize, respectively. In contrast, the high-density monoculture and strip-cropped maize showed peak performance under N300 and N375 fertilization rates. At both medium and high planting densities, the strip-cropped maize exhibited 8.9% and 10.9% lower breaking strength than the monoculture maize under N225 treatment. However, increasing the N fertilizer application resulted in comparable lodging resistance between the strip-cropped maize and the maximum values of the monoculture maize, at N300 treatment for medium density and N375 treatment for high density. Hence, strip-cropped maize planted at high density (75,000 plants/ha) with a lower nitrogen rate had lower lodging resistance than monoculture maize, but it can be improved to match the monoculture maize by increasing the nitrogen rate.