Biochar amendment modulate microbial community assembly to mitigate saline-alkaline stress across soil depths.

While microbial community assembly in saline-alkali topsoils is well-documented, distribution patterns across biochar application depths and soil layers remain unclear. This incubation study evaluated five treatment: no biochar (CK), homogeneous application (EB), and concentrated applications in upper (FB: 0-10 cm), middle (MB: 10-20 cm), or bottom layers (DB: 20-30 cm). Biochar application significantly accelerated vertical salt migration, with FB inducing 45.55 % and 61.01 % increases in water-soluble Na and Cl accumulation in the bottom layer. Microbial network complexity and interspecies interactions were highest in the upper layer (edges: 926), contrasting sharply with simplified communities in deeper layer (edges ≤552). Community assembly across layers was primarily driven by salt gradients, with deep-layer communities dominated by salt-tolerant taxa (such as Halomonas and Desulfobacterota). Among treatments, FB led to the highest biomarker abundance and α-diversity. Mechanistically, FB mitigated microbial diversity loss in mid-deep layers by establishing a symbiotic consortium of salt-tolerant keystone taxa (Bacillus-Pseudomonas-Ascomycota), which enhanced stress resilience via cross-feeding. These findings demonstrate that stratified biochar application (FB) optimizes salt redistribution while fostering stress-adapted microbial consortia across soil profiles, offering a targeted strategy for saline-alkali soil remediation.