Nano-engineered biochar effectively mitigates salinity stress in rice (Oryza sativa L.) through integrative physiological mechanisms

The escalating challenge of soil salinization necessitates the development of innovative and sustainable agricultural solutions. Recently, the integration of biochar and nanoparticles has emerged as a promising approach to mitigate salinity stress and enhance plant resilience. This study explored the effectiveness of rice straw biochar (20 t ha^-1) combined with ZnO and Fe₂O₃ nanoparticles (10 mg L^-1 and 20 mg L^-1) in improving the salt (EC ≈ 10.5 dS m^-1) tolerance of rice plants by examining the physiological and biochemical responses of two contrasting rice genotypes: JL534 (salt-sensitive) and XL900 (salt-tolerant). This study seeks to elucidate the underlying mechanism contributing to enhanced salt tolerance. A semi-field experiment was conducted, applying rice straw biochar (BC) at a rate of 20 t ha^-1 alone and in conjunction with ZnO and Fe₂O₃ nanoparticles at concentrations of 10 mg L^-1 and 20 mg L^-1. These treatments were designated as FeBC@1, FeBC@2, ZnBC@1, and ZnBC@2. The application of ZnBC@2 resulted in significant improvements in rice shoot biomass by 79.8 % (JL534) and 59 % (XL900), relative chlorophyll concentration by 68.2 %, 30.1 %, SPAD-index by 53.4 %, 35.2 %, relative water content by 60 %, 53.3 % and K+ content by 131.5 %, 76 % in JL534 and XL900, respectively. Additionally, ZnBC@2 increased the tolerance index and mitigated oxidative damage caused by salinity-induced over accumulation of ROS by reducing H₂O₂ and MDA levels while enhancing the activities of antioxidant enzymes. In contrast, high rate of Fe₂O₃ (FeBC@2) with biochar exhibits a notably lower response compared to lower rate of Fe₂O₃ (FeBC@1) due to its hormesis effect. In summary, this study highlights the potential of biochar and nanoparticles combination in enhancing plant salt tolerance.