Effects of Calcium Nutrition on Soybean Growth and Symbiotic Nitrogen Fixation

Calcium is essential for legume symbiotic nitrogen fixation, acting as both a nutrient and a signal. Yet, how varying calcium levels—from deficiency to toxicity—affect the soybean ‘root-nodule-stem’ balance has not been fully elucidated. To investigate this mechanism, a two-year sand culture experiment was conducted with three treatments: low calcium (0.1 mmol/L), moderate calcium (1 mmol/L), and high calcium (10 mmol/L), to systematically analyze their effects on soybean plant growth, nitrogenase activity, and nitrogen fixation capacity. The results indicated that the moderate calcium treatment supported the best root growth and nodule development, with both leghemoglobin (Lb) content and specific nitrogenase activity (SNA) reaching their peak levels. Low calcium stress significantly inhibited root elongation, while poor nodule development accompanied by a decrease in Lb content, thereby suppressing nitrogen fixation potential. In contrast to the low calcium treatment, although high calcium treatment inhibited root growth, it significantly increased the allocation of total plant dry matter to the root system. Under high calcium treatment, the ureide content in nodules increased significantly, whereas the ureide content in stems decreased substantially. This distributional imbalance suggests that high calcium obstructed the long-distance transport of nitrogen fixation products, subsequently leading to a significant decline in nitrogenase activity through a negative metabolic feedback mechanism. Calcium deficiency primarily resulted in structural impairments in nodule development, whereas high calcium induced functional inhibition by blocking ureide transport. Maintaining calcium homeostasis is important for ensuring efficient nitrogen fixation and source-sink balance in soybeans.