Green-synthesized zinc oxide nanoparticles and walnut biochar synergistically mitigate soil salinity and improve maize stress physiology

Introduction Soil salinization constrains maize productivity by elevating osmotic stress, disturbing nutrient homeostasis, and intensifying oxidative injury. In this study, we developed a green nano-biochar approach by synthesizing zinc oxide nanoparticles (ZnO NPs) from Coriandrum sativum leaf extract and combining them with walnut shell biochar (BC) to form a composite soil amendment, which may increase the nutrients (N, P, K) and increase plant antioxidant defenses in saline soil. Material and methods ZnO NPs were verified as crystalline ZnO with wurtzite structure and nanoscale morphology using Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy (SEM-EDS), Fourier Transform Infrared (FTIR), and X-ray Diffraction (XRD). A factorial pot experiment (15 days) was conducted in moderately (S-1; 4–5 dS m -1 ) and highly saline (S-2; 9–10 dS m -1 ) soils to compare ZnO NPs, BC, and composite against the unamended control. Results and discussion Additionally, maize plants treated with the composite demonstrated improved morphological traits, including a 43% increase in shoot, 41% higher total chlorophyll content, and a 28% increase in antioxidant enzyme activity. Stress diagnostics further showed improved membrane stability, with lower reactive oxygen species (ROS) burdens and reduced peroxidation and EC under saline conditions. Across both salinity levels, the composite consistently outperformed single amendments, underscoring the value of integrating ionic buffering with micronutrient delivery. By coupling plant extract synthesis and valorization of walnut residues, this study offers a resource-efficient alternative to conventional salinity management and offers a promising approach for potential crop productivity and soil health in saline-affected agricultural lands. However, studies should be done on the large scale in experimental fields.