European Journal of Sustainable Agroecosystems

Background: The process of converting atmospheric nitrogen into plant-usable forms incurs a carbon cost. Current carbon costs associated with symbiotic nitrogen fixation (SNF) typically range from 3 to 12 grams of carbon consumed per gram of nitrogen produced. Two important agricultural approaches represent extremes of the resulting carbon-to-nitrogen ratio. The first involves nano-formulated micronutrients that supply the enzymes and antioxidants required for efficient nodulation. The second relies on plant growth regulators (PGRs), particularly cytokinins, which control the number of nodules formed and thereby the quantity of nitrogen fixed by the plant.
Objective This review examines a specific and readily testable question that has received limited attention: whether the combination of nano-formulated micronutrients and plant growth regulators produces a greater amount of fixed nitrogen per unit of carbon than either intervention alone.
Method The analysis focuses on quantitative indicators such as molybdenum status and on published comparisons of individual and combined treatments. Key examples include the application of MoSâ‚‚ nanoparticles and the joint use of gibberellin A3 with mepiquat. Emphasis is placed on whether studies have employed identical factorial designs at the nodule level, incorporated carbon accounting, or reported isotopic tracer data.
Result A single application of MoSâ‚‚ nanoparticles at 10 mg kg⁻¹ increases soybean SNF and yield by approximately 30 % relative to conventional molybdate, by supplying the necessary cofactor and delaying nodule senescence. The clearest evidence of synergistic benefit arises from PGR–PGR combinations: gibberellin A3 promotes nodule development without suppressing nitrogenase activity, an inhibition that can be further mitigated by concurrent use of mepiquat. No study identified to date has directly compared nano-micronutrient versus PGR treatments within the same factorial design at the nodule level while accounting for carbon costs and reporting isotopic tracer measurements.
Conclusion Although individual nano-micronutrient and PGR interventions demonstrably enhance nitrogen fixation, rigorous evidence that their combination yields a superior fixed-nitrogen-to-carbon ratio remains lacking. Future factorial experiments that integrate nodule-level measurements, carbon budgeting, and isotopic tracing are required to resolve this question.