Low-temperature-pyrolysis biochar and ZnO nanoparticles improve growth and zinc uptake of maize (Zea mays L.) by altering root traits and rhizomicrobiome in sandy soil

Biochar (BC) and zinc oxide nanoparticles (nZnO) are promising amendments for improving crop performance in infertile soils. However, their combined effects on plant Zn acquisition and soil-plant-microbe interactions remain largely elusive. A pot experiment was conducted to investigate the effects of maize-straw BC pyrolyzed at 450 °C (LTBC) or 800 °C (HTBC), with or without nZnO addition, on maize growth, Zn accumulation, root traits, soil properties, and rhizosphere microbial communities in sandy soil. BC and nZnO induced contrasting root-trait responses. BC increased root tissue density but decreased root nitrogen concentration and specific root length, whereas nZnO showed the opposite responses. BC also enhanced photosynthetic performance and biomass accumulation, with more pronounced under LTBC than HTBC; whereas nZnO exerted comparatively weaker effects on these traits. The combined application of LTBC and nZnO significantly enhanced root and shoot zinc concentration by 244.5 and 172.2%, respectively, while resulting in highest Zn accumulation, relative to untreated control. BC enhanced soil organic carbon and microbial biomass carbon but reduced soil pH and microbial biomass nitrogen, whereas nZnO decreased microbial biomass carbon:nitrogen ratio. Moreover, BC modified bacterial community composition by increasing the relative abundances of Proteobacteria and Actinobacteriota , while suppressing Acidobacteriota , and co-application with nZnO enriched Aspergillus relative abundance. Taxonomy-based functional predictions suggested that treatment-associated shifts in putative bacterial functional profiles. Collectively, LTBC combined with nZnO showed potential for improving maize growth by integrating Zn supplementation with BC-mediated soil modification. These findings provide a basis for further evaluation of BC-assisted nano-Zn management in nutrient-poor sandy soils.