Nano-boron nitride enhances soybean growth and symbiotic nitrogen fixation by modulating the rhizosphere microbiome and biogeochemical cycling.

Nanotechnology offers promising strategies for sustainable agriculture, yet the systemic mechanisms by which nanomaterials enhance legume nitrogen fixation remain insufficiently understood. The multi-scale impacts of nano-boron nitride (nano-BN) on soybean growth, biological nitrogen fixation, and rhizosphere microbial communities were investigated. Soil addition of 50 mg kg-1 nano-BN significantly promoted plant biomass, nodule biomass, and leghemoglobin content by 10.0%, 27.4%, and 39.4%, respectively, compared to the untreated control. Nano-BN also enhanced the NH4+-N and NO3--N content by 24.3% and 19.9% in root tissues, while reducing these levels in rhizosphere soil. Additionally, nano-BN enriched rhizosphere-dissolved organic matter, particularly humic-like components. Metagenomic analysis revealed that nano-BN reshaped carbon and nitrogen cycling functional genes, enhancing CO2 fixation and aerobic respiration; the nitrogen fixation functional gene nifH was upregulated by 27.7%. Microbial community analysis demonstrated increased bacterial diversity and abundance of beneficial taxa, particularly Bradyrhizobium, which increased by 24.9%. Co-occurrence network analysis revealed enhanced positive interactions and greater topological complexity upon the addition of nano-BN compared to the untreated control, indicating improved community stability. Collectively, these findings demonstrate that nano-BN promotes soybean growth through integrated regulation of nutrient cycling, symbiotic nitrogen fixation, and rhizosphere microbiome assembly. Nano-BN represents an innovative nano-fertilization strategy for enhancing biological nitrogen fixation, improving nutrient use efficiency, and advancing sustainable agricultural systems.