Divergent carbon sequestration pathways in saline-alkali soils: Dual mechanisms of macroaggregate protection and chemoautotrophic compensation mediated by composted fermented straw amendments

Soil carbon sequestration mechanisms in saline-alkali ecosystems remain poorly understood, limiting precision management of organic amendments. We investigated composted fermented straw return (CFSR) versus conventional straw incorporation across salinity gradients (1.76 ​‰ vs. 4.06 ​‰) via controlled pot experiments. In lightly saline-alkali soils, CFSR elevated carbon accrual by 0.311 ​t ​ha⁻¹ yr⁻¹ (p ​< ​0.05) via macroaggregate formation (+4.19 ​% mass proportion) and Acidobacteriota enrichment. Conversely, heavily saline-alkali soils exhibited CFSR-induced microaggregate stabilization (+29.32 ​% stability index) coupled with chemoautotrophic carbon fixation, supported by 48.38 ​% higher cbbL gene abundance. Microbial network analysis revealed salinity-dependent adaptations that lightly saline soils under CFSR developed Acidobacteriota-dominated networks with elevated connectivity (graph density: 0.269 vs. 0.202 control), while extreme salinity fostered resilient Actinobacteriota-centric consortia (622 edges vs. 562 control) through modular simplification. Structural equation modeling delineated dual pathways that dissolved organic carbon-mediated macroaggregate stabilization dominated in light salinity (λ ​= ​1.902, p ​< ​0.05), whereas chemoautotrophic carbon pump-driven microaggregate protection prevailed under high salinity (λ ​= ​1.856, p ​< ​0.05). These findings established a hierarchical framework linking aggregate architecture to microbial functional guilds, proposing a dual-mode carbon stabilization paradigm — physical protection in light salinity versus microbial-mineral interactions under heavy salinity. This mechanistic insight advanced salinity-adaptive organic amendment strategies to optimize carbon storage in global salt-affected croplands.