Nitrogen Fertilization and Planting Density Mediate Wheat Yield Formation and Resource-Use Strategies Along the Plant Economics Spectrum in Two Single-Season Field Experiments

Nitrogen fertilization and planting density are key regulators of wheat population growth and yield; however, few studies have examined their combined effects on multi-trait resource trade-offs via the plant economics spectrum. Two single-season split-plot field experiments were conducted in the 2024–2025 cropping year using the winter wheat cultivar Womai106 and the spring wheat cultivar Jinchun6 under three nitrogen rates and five density gradients. Owing to their divergent sowing and harvesting phenology, the two cultivars were tested in separate growing seasons. We quantified biomass, yield components, photosynthetic and canopy traits at anthesis, and leaf C:N:P stoichiometry, and applied redundancy analysis, the log response ratio, and PCA to elucidate nitrogen and density regulation of wheat growth, nutrient allocation, and plant economics spectrum strategies. Nitrogen dominated wheat growth, leaf nutrition, and yield potential, whereas density modified canopy structure and population-level biomass and yield components in both cultivars, as well as photosynthetic rate in Jinchun6. Nitrogen × density interactions were significant for only one of the traits assessed (leaf area index in Womai106) and were not significant for any other trait. Grain yield followed a unimodal quadratic trend along the density gradients, although the quadratic term was significant at all nitrogen levels in Womai106 but only under N2 in Jinchun6, and the fitted optima had widely overlapping confidence intervals among nitrogen levels. Grain number per unit area, rather than thousand-kernel weight, determined final yield. Leaf stoichiometry was primarily regulated by nitrogen and showed strong homeostasis under crowding, with leaf [P] and leaf [C]:[P] in Womai106 being the exception. Plant economics spectrum scores shifted along the density gradient in both cultivar × season combinations, and the direction of this shift differed among nitrogen levels; in Jinchun6, acquisitive trait values were associated with high density only under high nitrogen. These findings provide a basis for ecotype-specific nitrogen and planting density management, and warrant confirmation across years, sites and additional genotypes.