Silicon Nanoparticles Alter Soybean Physiology and Improve Nitrogen Fixation Potential Under Atmospheric Carbon Dioxide (CO).

The interactive effects between nano-silicon dioxide (-SiO) and elevated CO (eCO; 645 ppm) on soybean physiology, nitrogen fixation, and nutrient dynamics under climate stress remain underexplored. This study elucidates their combined effects under ambient (aCO; 410 ppm) and eCO conditions. eCO + -SiO synergistically enhanced shoot length (30%), total chlorophyll (112.15%), and photosynthetic rate (103.23%), alongside improved stomatal conductance and intercellular CO (17.19%), optimizing carbon assimilation. Nodulation efficiency increased, with nodule number and biomass rising by 48.3% and 53.6%, respectively, under eCO + -SiO versus aCO. N-assimilation enzymes (nitrate reductase, nitrite reductase, glutamine synthetase, glutamate synthase) surged by 38.5-52.1%, enhancing nitrogen metabolism. Concurrently, phytohormones (16-21%) and antioxidant activities (15-22%) increased, reducing oxidative markers (18-22%), and bolstering stress resilience. Nutrient homeostasis improved, with P, K, Mg, Cu, Fe, Zn, and Mn elevating in roots (13-41%) and shoots (13-17%), except shoot Fe and Zn. These findings demonstrate that -SiO potentiates eCO-driven benefits, amplifying photosynthetic efficiency, nitrogen fixation, and stress adaptation through enhanced biochemical and nutrient regulation. This synergy underscores -SiO role in optimizing crop performance under future CO-rich climates, advocating nano-fertilizers as sustainable tools for climate-resilient agriculture.