Pre-biodrying enhances compost humification via redirecting carbon allocation towards microbial necromass formation

Pre-biodrying is considered to promote compost maturation and humification by accelerating extracellular organic matter degradation, but how it affects subsequent microbial carbon assimilation and carbon allocation remains unclear. This study evaluated the effects of pre-biodrying on plant-derived carbon (PDC) transformation, microbial necromass carbon (MNC) accumulation, and humification during the composting of solid dairy manure digestate. Results showed that pre-biodrying significantly improved compost maturity and humification degree, as indicated by higher germination index and degree of polymerization at 15 d (Humic acid carbon = 37.31 ± 0.68, Degree of polymerization = 3.50 ± 0.43; P < 0.001). Carbon-pool analysis showed that pre-biodrying composting (BD) not only accelerated PDC depletion, but also promoted MNC accumulation. By 15 d, PDC was lower in BD than in traditional single-stage composting (TC), whereas bacterial necromass carbon (17.23 ± 1.11 vs. 14.47 ± 0.83 g kg^-1) and fungal necromass carbon (20.37 ± 1.85 vs. 18.23 ± 1.78 g kg^-1) were significantly higher in BD. DOM fluorescence characteristics further showed a higher proportion of microbially derived fluorescent components in BD. Microbial analysis indicated that BD enriched microbial taxa associated with complex carbon transformation (Streptomyces and Microascus) corresponding to MNC accumulation. Mechanistic analysis suggested that pre-biodrying enhances digestate compost humification by promoting microbial carbon assimilation and turnover, thereby redirecting carbon allocation towards microbial necromass formation and providing more microbially derived carbon that may contribute to humic acid formation and stabilization.