Combining Ferroferric Oxide with a Compound Microbial Agent to Accelerate Humification during Agricultural Waste Composting

The slow decomposition of lignocellulose is a major challenge in the aerobic composting of agricultural waste. Reactive oxygen species (ROS) generated in an aerobic–anoxic interface during composting can attack the lignocellulose structure to accelerate humification. This study attempted to produce more ROS by combining Fe3O4 with a compound microbial agent (CMA; B. subtilis and P. chrysosporium) (CMA-Fe) to drive the Fenton-like reaction to accelerate composting. The results revealed that the •OH concentration of the CMA-Fe group was 0.084 mmol/kg, which was higher than that of the CMA group and the control group without CMA and Fe3O4. Accordingly, the thermophilic phase duration of the CMA-Fe group was extended to 14 days, which was beneficial for the humification. The germination index, indicating the maturity of the compost, attained 70.72% by day 20 in the CMA-Fe group, which was significantly shorter than that of the other two groups. The analysis of the humic acid content and the cyclic voltammetry of the compost sample indicated that the addition of CMA and Fe3O4 accelerated the humification. Furthermore, Fe3O4 enriched iron-reducing bacteria capable of participating in microbially driven Fe(III)/Fe(II) redox transformations. Thus, combining iron oxides and CMA might provide a novel approach for shortening the composting period and improving the quality of compost products.