Interactive effects of calcium-nitrogen combined stress and nitrogen forms on nitrogen metabolism and physiological ecological response mechanisms of Toona sinensis Seedlings

can achieve adaptive growth under normal calcium conditions by adjusting nitrogen utilization strategies, whereas irreversible physiological collapse occurs under high calcium stress. High calcium acts as a limiting factor by disrupting redox balance, nitrogen metabolism, and ion homeostasis. In karst regions, a nitrate-to-ammonium ratio of 1:1 is recommended for fertilization under normal soil calcium conditions; however, in high-calcium habitats, priority should be given to reducing calcium levels rather than adjusting nitrogen levels or forms.