Genotype-Specific Rhizosphere Microbiome Assembly Mediates Biochar-Induced Salt Tolerance in Sorghum

Sorghum genotypes differentially shape their rhizosphere microbiomes to cope with salt stress; however, the modulatory role of biochar in this genotype-specific plant–microbe interplay remains unclear. In this study, we investigated how salt-sensitive (Henong 16, HN16) and salt-tolerant (Jizaonuo 1, JZN) sorghum genotypes leverage biochar to assemble distinct functional rhizosphere microbiomes under salt stress (5 g kg−1 NaCl). Biochar application (20 g kg−1) alleviated ionic stress by reducing soil electrical conductivity (EC decreased by 46% in HN16) and enhanced soil fertility through increased organic matter (SOM increased by 26% in JZN). 16S rRNA gene sequencing revealed that biochar selectively enriched genotype-specific, stress-resistant taxa. The salt-sensitive HN16 primarily recruited Sporosarcina (a genus reported to exhibit salt tolerance and nitrogen-fixing capabilities) and Intrasporangiaceae, thereby rapidly establishing a rhizosphere barrier. In contrast, the salt-tolerant JZN consistently enriched Salinimicrobium (an extreme halophile) and the family LWQ8, forming more complex and stable co-occurrence networks with a higher proportion of positive correlations (81%). Plant genotype was the primary determinant of core microbiome assembly: Bacillus and Arthrobacter dominated in HN16, whereas Sphingomonas and Streptomyces prevailed in JZN. Biochar reinforced this genotype-specific assembly by modulating soil pH and SOM, which were identified as key drivers of microbial community divergence. Importantly, these biochar-shaped microbial modules showed significant positive correlations with increased plant biomass. Our findings demonstrate that biochar enhances salt tolerance not merely by improving soil properties, but primarily by facilitating the deterministic assembly of genotype-specific, functional rhizosphere microbiomes. This mechanistic insight shifts the paradigm of biochar from a universal soil amendment to a precision tool for rhizosphere engineering, providing a genotype-aware foundation for enhancing salinity resilience in sustainable agriculture.