Karst habitats are characterized by composite stress conditions featuring high calcium, low nitrogen, and a unique nitrogen form profile ("low ammonium, high nitrate"), which profoundly shape plant physiological adaptation strategies. Current research often examines the isolated effects of either calcium or nitrogen, failing to adequately explain their interactions in karst environments. Therefore, investigating the interactive effects of nitrogen level and form under high calcium stress on the growth and physiological traits of Toona sinensis seedlings is crucial for understanding plant adaptation mechanisms in this region. A hydroponic experiment was conducted to investigate the interactive effects of nitrogen level and form on the growth and physiological characteristics of Toona sinens is seedlings under high calcium stress. Two calcium levels (5 mM and 120 mM), three nitrogen levels (3.75, 7.5, and 15 mM), and three nitrate-to-ammonium ratios (0:100, 50:50, and 100:0) were applied. Morphological, physio-biochemical, and mineral element indices were measured. The results showed that under normal calcium conditions, the mixed nitrogen form (50:50) was optimal at medium and high nitrogen levels. However, when the calcium concentration was increased to 120 mM, high-calcium stress completely overrode physiological responses and disrupted core metabolic functions, leading to a collapse of physiological functions in the seedlings. Specifically, a burst of reactive oxygen species (ROS) exacerbated membrane lipid peroxidation, with root malondialdehyde (MDA) content surging to 7.68 times that observed under normal calcium conditions. The antioxidant system (SOD, POD) became dysfunctional, and the activities of nitrogen-metabolizing enzymes (NR, GS) were almost completely inhibited, thereby hindering the conversion of inorganic nitrogen to organic nitrogen. Simultaneously, mineral ion homeostasis (e.g., Ca/Mg ratio) was severely disturbed, resulting in the breakdown of metabolic homeostasis. These findings indicate that T. sinensis 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.
Hang et al. (Fri,) studied this question.