Nanoplastic and nickel nanoparticles modulate soybean performance and nitrogen assimilation

The increasing release of nanoplastics (NPx) and engineered nanoparticles (NPs) into terrestrial ecosystems poses emerging risks to soil biota and the food chain. The effects of NPx contamination on soybean growth and N₂ fixation potential in the presence of nickel oxide nanoparticles (NiO-NPs) remain poorly understood. This study investigated the accumulation and phytotoxicity of polystyrene nanoplastic (PS NPx) and NiO-NPs in soybean and their effects on nitrogen fixation potential. Single or co-application of NPx and NiO-NPs (100 mg kg^-1) markedly reduced photosynthetic efficiency (8-48%), elevated biochemical stress markers (25-164%), downregulated phytohormone levels (13-54%), and negatively affected nitrogen fixation potential (19-35%), ultimately leading to reduced plant physiological growth (21-57%). This was due to PS-NPx accumulation in the leaves and structural abnormalities in vacuoles (including altered vacuole morphology), cell walls, mitochondria, and chloroplasts. In contrast, NiO-NPs at 50 mg kg^-1 significantly increased nodule formation (16%) and biomass (20%), thereby enhancing N₂-fixation (14%) in soybean plants. This improvement was attributed to the upregulation of N₂ assimilation enzymes in nodules, roots, and shoots that promoted N₂ content in the plant, resulting in increased plant growth. This study improves our understanding of the potential environmental risks associated with the accumulation of NPx and NiO-NPs in legumes crops and provides valuable scientific insights into their impacts on plant growth and N₂ fixation in soybean.