Contrasting soil carbon stabilization pathways following spent mushroom substrate valorization into compost,...

Enhancing soil organic carbon (SOC) through exogenous organic materials (EOMs) is an important strategy for improving soil carbon storage. However, how EOM properties and initial soil SOC levels jointly regulate carbon retention remains unclear. Therefore, a 120-day microcosm experiment was conducted using two purple soils with contrasting SOC levels (high-SOC soil, 20.9 g kg^-1; low-SOC soil, 5.92 g kg^-1) amended with spent mushroom substrate (SMS) and its derived compost, hydrochar, and biochar. Results indicated that under equivalent carbon inputs, carbon sequestration efficiency generally followed the order biochar > hydrochar > compost > SMS in both soils and was numerically higher in low-SOC soil across all EOM treatments. Specifically, biochar achieved the greatest increase in measured SOC and preferentially increased particulate organic carbon (POC). Hydrochar preferentially increased mineral-associated organic carbon (MAOC) fraction without a comparable increase in microbial necromass. In contrast, SMS and its derived compost strongly enhanced microbial biomass, carbon use efficiency, and microbial necromass accumulation. These responses were further modified by initial soil SOC levels: low-SOC soil showed stronger SOC accumulation and carbon-retention responses, whereas high-SOC soil showed comparatively smaller SOC gains but pronounced microbial processing and necromass accumulation. Collectively, these findings demonstrate that SMS valorization products differ in both carbon-retention efficiency and SOC partitioning, and that their effectiveness depends on the receiving soil context, supporting soil-specific selection of organic amendments for carbon management. These findings provide a mechanistic basis for integrating SMS valorization with soil-specific carbon management, thereby linking organic-waste recycling with soil carbon sequestration and more sustainable agricultural systems.