One-step microbial conversion of waste molasses into bioactive fulvic-like acid fertilizer for sustainable soil quality improvement and agricultural waste valorization

Waste molasses (WM), a carbon rich by-product of sugar production, is frequently underutilized, resulting in environmental pollution and resource loss. In this study, a one-step microbial bioconversion process using Bacillus subtilis was developed to transform WM into molasses derived fulvic acid (MFA). The highest MFA yield (33.54%) was achieved at 35 °C, 48 h, and a 5% inoculation rate. Integrated GC-MS and LC-MS analyses revealed dynamic substrate transformation during fermentation and suggested that 4-hydroxy-2-methylacetophenone, 4-oxododecanedioic acid, and 3-hydroxy-2-octylpentanedioic acid were putative intermediates involved in MFA formation. Structural characterization showed that the resulting MFA possessed a loose, porous morphology and abundant hydroxyl, carboxyl, and carbonyl groups. In a single 150 d field trial, MFA application was associated with higher soil organic carbon and humus contents than the unfertilized control and promoted maize vegetative growth. MFA also enriched the microbial communities including Pseudomonas , Acidobacterium , Actinobacteria , and Bacteroidetes . The abundance of several genes related to carbon assimilation and carboxylation, including rbcL , rbcS , accB , accC , and pccA , has also increased. Metabolomic analysis further revealed shifts in metabolites associated with central carbon metabolic pathways, including glycolysis and the pentose phosphate pathway MFA application also markedly promoted maize growth without chemical fertilizer input, demonstrating its potential as an organic soil amendment and partial fertilizer substitute. Collectively, these findings suggest that a one-step microbial transformation can convert WM into functional fulvic-like acid fertilizer. This method provides a potential route for mineral derived FA production. Simultaneously combining the reuse of agricultural waste, improvement of soil quality, reduction of partial fertilizer dependence, and development of circular agriculture.