Hydroxyapatite-Modified Biochar Application Derived from Recycled Selenium-Rich Straw Enhances Rice Biofortification and Economic Benefits

Agricultural straw serves as a critical sink of soil selenium (Se) that cannot be overlooked, necessitating resource-oriented utilization guided by Se recovery to align with sustainable development goals. This study developed a novel strategy of hydroxyapatite-modified biochar (PBC) application derived from Se-enriched straw for enhanced rice Se and economic benefits through cross-scale investigations encompassing soil incubation, pot experiments, and field trials. Results indicated that rice straw exhibited both a higher Se enrichment factor (59.3%) from soil and concentration (0.211 mg kg^-1) than grains. Application of 1% (w/w) PBC (modified at 2% concentration (w/w)) maximally increased soil-available Se and grain Se content by 47.2% and 55.6%, respectively, compared to the control. The soil Se mobilization mechanism involved three synergistic pathways: intrinsic Se release, adsorption suppression, and soil Se activation. Subsequently, PBC augmented Se sequestration in root iron plaques through enhanced iron deposition, then upregulated expression of transmembrane transporter genes (OsSULTR1;2, OsPT2, OsNIP2;1, and OsSULTR2;1), facilitating Se uptake and translocation. Economically, PBC application generated net benefits of US$18,301 per hectare, representing US$1,137 and US$16,476 increases over BC and control, respectively. This study establishes a novel technological framework for closed-loop Se resource recovery in agroecosystems while delivering actionable insights for sustainable development.