Interactive Effects of Calcium-Nitrogen Combined Stress and Nitrogen Forms on Nitrogen Metabolism and Physiological Ecological Response Mechanisms of Toona sinensis Seedlings

Toona sinensis can achieve adaptive growth by adjusting its nitrogen utilization strategy under a normal calcium environment, while irreversible physiological collapse occurs under high-calcium stress. High calcium, by disrupting redox balance, interfering with nitrogen metabolism and ion homeostasis, becomes a key factor limiting the early establishment of seedlings. This study concludes that in normal-calcium soils in karst areas, a mixed fertilization with a nitrate-ammonium ratio of 1:1 can be adopted to optimize nitrogen efficiency and enhance stress resistance; whereas in habitats under high-calcium stress, priority should be given to alleviation measures such as calcium reduction or passivation to restore the basic metabolic functions of plants.