Effect of Nitrogen Application Timing on the Photosynthetic Traits and Essential Oil Yield of Cinnamomum camphora var. linalooliferum Coppice Stands

To investigate the effects of nitrogen application timing on photosynthetic traits and essential oil yield of Cinnamomum camphora var. linalooliferum coppice stands, an experiment was conducted with different nitrogen allocation ratios at coppicing, topdressing in June and August using a constant nitrogen amount of 270 kg·ha−1. Plant growth, photosynthetic traits, and total above-ground essential oil yield were measured dynamically. Nitrogen application timing significantly affected photosynthetic performance, biomass accumulation, and essential oil yield (p < 0.05), showing clear stage-specific responses. It was found that early growth favored higher basal nitrogen fertilization, whereas middle-to-late growth benefited from increased topdressing. Principal component analysis revealed strong coordination among photosynthesis, growth traits, and essential oil yield, with N5 closely associated with high essential oil yield per plant (TEOyp), plant height (PH), leaf area index (LAI), relative chlorophyll value (SPAD), and biomass traits. Correlation analysis further confirmed the stage-specific regulatory effects of photosynthetic traits and plant growth on essential oil yield. Nitrogen fertilization at coppicing mainly enhanced photosynthetic efficiency to support essential oil formation, whereas topdressing promoted vegetative growth and biomass accumulation to exploit yield potential. These findings will provide theoretical support and practical guidance for efficient nitrogen management in high-yield cultivation.