Effect of iron-modified nanobiochar on plant growth, redox homeostasis and soil bacterial diversity in Apocynum pictum under salinity stress

Soil salinity reduces crop production, yet current mitigation strategies like conventional biochar are limited by their passive mode of action and inability to address salinity-induced nutrient deficiencies, particularly iron deficiency. Here, we hypothesized that an iron-doped nanobiochar (FeBC) would outperform unmodified simple nanobiochar (SBC) by actively supply iron, reducing sodium accumulation and restructuring the rhizosphere microbiome under salinity. To test this, we applied FeBC and SBC for evaluating their efficacy in mitigating 300 mM NaCl stress in the fiber crop Apocynum pictum. Comparing biochars, FeBC (1% w/w) exhibited distinct physicochemical properties with 55% higher surface area, 38% smaller particle size, intrinsic acidity (pH 3.45 vs. 7.2 SBC), and nanocrystalline maghemite/hematite phases (≤10 nm) dispersed within the carbon matrix. These features enabled salinity-triggered Fe release via Na+ -Fe3+ cation exchange, a mechanism absent in SBC. Under salinity, FeBC restored shoot (86%) and root (122.9%) biomass than control, while SBC achieved only partial recovery. This was associated with FeBC induced higher root Fe (116%), reduced root Na+ (25%), and lowered oxidative damage, higher POD activity (94%), and total flavonoid content (+187.7%) under salinity. The 16S rRNA sequencing revealed that FeBC recruited a distinct bacterial consortium enriched in beneficial taxa, including Solirubrobacter and Blrii41 and increased soil microbial biomass carbon and catalase activity. Whereas SBC recruited a compositionally distinct community. Our results showed that FeBC outperformed SBC in enhancing salinity tolerance in A. pictum and suggest that FeBC could be used as a nano-enabled soil amendment strategy for cultivating salt-sensitive crops in saline soils.