ORIGINAL RESEARCH article
Front. Plant Sci.
Sec. Plant Symbiotic Interactions
Nickel treatment of soybean seeds: evaluating optimal levels for Bradyrhizobium spp. survival, nitrogen fixation, physiological traits and grain yield
Provisionally accepted
Luiz Gustavo Moretti1
Carlos Alexandre Costa Crusciol1*
Marco Antonio Nogueira2
João William Bossolani1
José Roberto Portugal1
Mariangela Hungria2- 1Universidade Estadual Paulista Julio de Mesquita Filho - Campus de Botucatu, Botucatu, Brazil
- 2Embrapa Soja, Londrina, Brazil
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Accumulating evidence indicates that fertilizing soybean with nickel (Ni) can enhance biological nitrogen fixation (BNF) and plant productivity. Seed application is ideal for promoting nodule formation and early plant development, but this practice raises the risk of toxicity by reducing the difference between beneficial and harmful doses. Unfortunately, studies of the effects of Ni on the survival of Bradyrhizobium spp. applied as inoculants or on BNF have not yielded a consensus on an optimal dose. The objective of this study was to establish Ni thresholds that maximize physiological and productivity benefits for soybean, balancing Ni's positive effects on BNF and plant growth against risks of phytotoxicity and bacterial inhibition. Soybean seeds were treated with nickel sulfate (NiSO₄·6H₂O) at six doses: 0, 60, 120, 180, 240, and 300 mg Ni kg⁻¹. We then assessed the effects of seed treatment with Ni on the recovery of Bradyrhizobium cells from treated seeds, BNF as assessed by continuous-flow analysis of acetylene reduction activity (ARA), and soil CO₂ evolution in greenhouse experiments. In addition, we evaluated the impact of Ni dose on the physiological, nutritional, agronomic traits, and grain yield of soybean in multi-site field trials over two cropping seasons. Ni doses of up to 60 mg kg⁻¹ enhanced nitrogenase activity, nodulation, nodule biomass, and grain yield without compromising Bradyrhizobium 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 (p < 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.
Keywords: Acetylene reduction activity, biological nitrogen fixation, Glycinemax (L.) Merrill, micronutrient, nodulation
Received: 30 Jun 2025; Accepted: 16 Dec 2025.
Copyright: © 2025 Moretti, Crusciol, Nogueira, Bossolani, Portugal and Hungria. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Carlos Alexandre Costa Crusciol
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