Dose-dependent effects of ferrous sulfate on carbon-nitrogen stabilization and microbial functional pathways in...

Although ferrous sulfate (FeSO₄) is increasingly applied in aerobic composting, its dosage-dependent effects on carbon stabilization, nitrogen retention and microbial functional pathways remain insufficiently defined. In this study, livestock manure composting was conducted with four FeSO₄ dosages (0, 0.5, 1.5 and 4.5%) to evaluate physicochemical dynamics, humification, microbial succession and predicted carbon-nitrogen functional profiles. Compared with the control, moderate FeSO₄ addition (0.5-1.5%) significantly accelerated thermophilic onset and improved compost maturity (P < 0.05). Within this range, 1.5% FeSO₄ achieved the highest humic acid accumulation and humification indices, together with significantly lower late-stage dissolved organic carbon, indicating enhanced carbon stabilization. Total nitrogen retention was also significantly higher under 0.5-1.5% FeSO₄ than in the control and 4.5% treatment. In contrast, excessive FeSO₄ input (4.5%) constrained humification and redirected carbon metabolism toward glycolytic and fermentative pathways. Functional prediction revealed that nirK/nirS abundance peaked at 1.5%, whereas higher FeSO₄ enriched norBC, nosZ and amoABC, indicating intensified terminal denitrification and nitrification potential. Redundancy analysis showed that carbon-related variables explained most variation in carbon- and nitrogen-cycling genes. Overall, 0.5-1.5% FeSO₄ represented the most suitable operational range for improving manure compost quality, with 1.5% providing the best integrated performance under the conditions tested.