Nitrogen–Calcium Stoichiometry Regulates Growth and Physiology in Mongolian Pine (Pinus sylvestris var. mongolica)

Nitrogen and calcium are the key elements required for plant growth. Variations in calcium concentration will affect nitrogen absorption in plants, regulate photosynthetic processes, and participate in the absorption and transport of photosynthetic products. The changes of nitrogen and calcium nutrients is conducive to alleviating the decline and mortality of Mongolian pine forests, thus contributing to the preservation of regional ecological security. In this study, an investigation was conducted into the effects of seven nitrogen-to-calcium (N-Ca) ratios (1:8, 1:4, 1:2, 1:1, 2:1, 4:1, and 8:1) on the growth and physiology of Mongolian pine seedlings through pot experiments. The results of the one-way analysis of variance indicated that variations in the N-Ca ratio could significantly affect processes such as plant height, basal diameter, biomass accumulation, and photosynthesis in Mongolian pine seedlings. A low N-Ca ratio caused calcium toxicity, resulting in reduced stomatal conductance (Ci) and a lower net photosynthetic rate (Tr). Conversely, a high N-Ca ratio led to nitrogen toxicity, decreased antioxidant enzyme activity, and adversely affected the accumulation of photosynthetic pigments and photosynthetic products. At an N-Ca ratio of 2:1, Mongolian pine seedlings not only exhibited maximized biomass and photosynthetic capacity but also demonstrated significantly elevated levels of antioxidant enzymes and content of soluble substances. In conclusion, an optimal N-Ca ratio of 2:1 existed for Mongolian pine seedlings, which significantly improved their growth and physiological characteristics.