Ferrous salts accelerate humification and reduced carbon emissions during the co-composting of hoggery slurry and wheat husks: New insights into their biotic and abiotic functions and mechanisms

In this study, the effects and mechanisms of water-soluble ferrous salts (WSFS) on the humification and carbon conversion during the co-composting of hoggery slurry (HG) and wheat husks (WH) were investigated. A composting mixture of HG and WH was treated with FeCl 2 addition (T1) and FeSO 4 addition (T2), and compared with an untreated control (CK), as a function of time (28 days). The duration of the high-temperature phase (>50 °C) for CK, T1, and T2 are 17, 15, and 15 days, respectively. Compared to the CK, T1 and T2 treatments reduced CO 2 emissions by 11.4 % and 32.0 %, respectively. The calculated total global warming potential values were 293.67 (CK), 267.69 (T1) and 207.10 (T2) g CO 2 eq kg −1 . The humus concentrations increased in all treatments: CK by 28.10 %, T1 by 53.41 % and T2 by 49.73 % in composting. WSFS addition enhanced biotic humification by elevated the abundance and distribution of Bacillus , which promotes cellulose and hemicellulose decomposition, and Tepidimicrobium , which accelerates the production of humus, through use of sugars and proteins. Besides, WSFS addition decreased the levels of pathogenic bacteria in the final compost product. Notably, WSFS addition also benefited the abiotic humification by production of •OH during oxidation of Fe(II) in composting. This study enhances the understanding of the biotic and abiotic functions of FeSO 4 and FeCl 2 in promoting humification and reducing carbon emissions during composting. The detailed role of hydroxyl radical in composting and the effectiveness of WSFS-addition in composting nitrogen transformation are still need to be addressed in future.