Inoculation with phosphate-solubilizing bacteria enhances humification during composting: Insights from molecular characterization and carbon metabolism

The inoculation of phosphate-solubilizing bacteria (PSB) is a recognized strategy for optimizing phosphorus cycling in composting; however, its regulatory mechanisms in driving the humification process remain poorly understood. This study comprehensively evaluated the transformation of dissolved organic matter and the intensification of humification during composting inoculated with Pseudomonas fluorescens (PSB_P) and Klebsiella variicola (PSB_K). The results demonstrated that at the end of the composting, the PSB_P group exhibited a substantial increase in humic acid (HA) content, with HA levels being 20.83% higher than those in the control group (CK). Spectroscopy and mass spectrometry further revealed that the PSB_P treatment led to a 2.1% increase in humic acid-like substances, accompanied by a significantly higher number of aromatic molecular formulas and increased structural complexity. Mechanistically, the inoculation of Pseudomonas fluorescens optimized the microbial community assembly, strengthening the dominance of the phylum Pseudomonadota. This shift significantly boosted carbon metabolism functions: on day 4, the abundance of lignin degradation functions in the PSB_P group was 8.26% and 5.78% higher than in the CK and PSB_K groups, respectively. Network analysis identified that the synergistic interplay between lignin degradation, lipid degradation, and amino acid utilization served as the primary driver for the synthesis of stable humic precursors. Moreover, the improved humification process in the PSB_P group also effectively suppressed carbon dioxide and methane emissions. Overall, this study provides molecular and genetic insights into the effects of PSB inoculation on composting humification.