Seed-Applied Cobalt, Molybdenum, and Nickel Improve Nitrogen Metabolism in Soybean Plants Across Seed Vigor Levels

Cobalt, molybdenum, and nickel are elements directly involved in biological nitrogen fixation in legume plants. However, there is a lack of information about the effects of the interaction among these elements on seed vigor and plant development. This study aimed to evaluate the effects of different doses of these elements on soybean seeds with higher and lower vigor, focusing on nitrogen metabolism and plant development under controlled conditions. The two lots of soybean seeds (higher and lower vigor) were treated with doses of 0, 2, 4, 6, and 8 mL kg^-1 of seeds of a liquid commercial product composed of cobalt, molybdenum, and nickel. At the full flowering stage, urease and nitrogenase activities, dry biomass of shoots, roots, and nodules, nitrogen concentration in shoots, plant height, number of nodules, and the efficiency of biological nitrogen fixation (measured by nitrogen-15 isotopic ratio) were assessed. Urease activity increased by 191% in high-vigor seed plants and 65% in low-vigor seed plants. Nitrogenase activity was higher in higher-vigor plants. Nodule dry biomass increased by 42% in lower-vigor plants compared to the control treatment, while in higher-vigor plants, it decreased with increasing doses. Shoot biomass was 30% higher than the control at the 2 mL kg^-1 dose. In general, the best responses to the application of the elements in the evaluated variables were observed with the doses of 2 and 4 mL kg^-1. It is concluded that the appropriate application of cobalt, molybdenum, and nickel on seeds enhances growth and symbiotic efficiency. However, excessive doses may cause phytotoxic effects.