Nickel treatment of soybean seeds: evaluating optimal levels for Bradyrhizobium spp. survival, nitrogen fixation, physiological traits and grain yield

Ni doses of up to 60 mg kg^-1 enhanced nitrogenase activity, nodulation, nodule biomass, and grain yield without compromising <i>Bradyrhizobium</i> viability. Doses exceeding this threshold reduced bacterial survival, nodulation, and yield, indicating Ni toxicity. The field trials exhibited a natural gradient in soil Ni levels and texture (0.4-0.6 mg dm^-3 sandy to clayey), which helps explain differences in response magnitude and reinforces the need for contextualized recommendations. Applying a second-order polynomial regression to mean standardized Z-scores of integrated agronomic traits revealed a significant quadratic response (<i>p</i> < 0.05). Consequently, an agronomically optimal range of 50-100 mg Ni kg^-1 is recommended to sustainably optimize soybean growth and N fixation by balancing the benefits and risks of Ni application.