Soil nitrogen availability determines the CO₂ fertilization effect on tree species (Neolamarckia cadamba): growth and physiological evidence

Urban plantation species experience multiple stresses. Among these, the two significant challenges are increasing atmospheric CO₂ concentration and nutrient-depleted soils, which significantly impede growth, development, as well as the adaptation and mitigation capacity of plant species. This study hypothesized whether nitrogen availability improves CO₂ fertilization effects and the growth of urban plantation tree species (Neolamarckia cadamba) under rising atmospheric CO₂ concentration.The plants were grown in nitrogen regimes (low-N₂₀₀ Kg N ha⁻¹, medium-N₃₀₀ Kg N ha⁻¹, and high-N₅₀₀ Kg N ha⁻¹) under elevated CO₂ concentration (eCO₂; 800 ± 20 µmolCO₂ mol⁻¹) and ambient conditions (aCO_2; 400 ± 14 µmolCO₂ mol⁻¹). We reported that growth and physiological traits were significantly improved under elevated CO₂ concentration and applied nitrogen compared to low nitrogen and ambient CO₂ concentration. The height, stem diameter, leaves, leaf area, and branches were increased by 25%, 13%, 12%, 6%, and 21%, respectively, under N₃₀₀ and eCO₂ than counterparts. The leaf CO₂ assimilation, transpiration, and stomatal conductance were enhanced by 17%, 39%, and 57%, respectively, whereas water use efficiency declined under N₃₀₀ and eCO₂ but slightly increased in eCO₂ and N₅₀₀. We inferred that nitrogen management practices would improve the benefits of rising atmospheric CO₂ concentration, resulting in improved plant growth and development and better adaptive physiological response and mitigation potential of plantation species.