The co-addition of biochar and manganese ore promotes nitrous oxide reduction but favors methane emission in sewage sludge composting

In the present study, the effects of biochar and manganese ore (MO) on N2O and CH4 emissions during sewage sludge composting were investigated. The results demonstrated that the additive additions could effectively reduce total nitrogen loss and improve the agronomic value of compost. Furthermore, all the additive treatments could mitigate N2O (6.60–45.9%) and CH4 (33.6–71.3%) emissions. Compared to the single corn straw biochar (CSB) or MO addition, the co-addition of CSB and MO can further promote N2O reduction by 14.6–22.4% but increase CH4 emission by 22.9–33.7%. Overall, the co-addition of CSB and MO showed the lowest global warming potential, which was 42.1% lower than the control, among all the additive treatments. Integrating the analyzed results of Spearman rank correlation coefficients and functional genes, the inhibition of N2O generation during nitrification and denitrification processes was identified as the main mechanism for the reduced N2O emissions. For CH4, the reduced CH4 emissions were contributed by simultaneously inhibiting CH4 generation and promoting CH4 consumption, especially for the single CSB or MO treatments. Additionally, based on the analyzed results of the structural equation model, amoA and nirS were found to be the key functional genes affecting N2O emissions directly, and temperature and pH were identified as the root factors affecting N2O emissions through varied biological or chemical pathways. These findings could provide a new alternative to produce high-quality sewage sludge compost with lower environmental impact in an economical way.