Iron-modified hydrochar mitigates N2O emissions in rice soils by inhibiting nitrification and enhancing complete denitrification: Comparative between hydrochar and pyrolyzed char

Iron-modified biochar was extensively investigated for use in ecological remediation. The emission of nitrogen- containing greenhouse gases, driven by the excessive application of N fertilizers, has become a critical factor affecting the atmospheric environment. Nevertheless, the impact of iron-modified biochar as a soil amendment with exceptional performance on N2O emissions remains poorly understood. In this paper, pyrolytic iron- modified biochar (700Fe@RC) modified by Fe(NO3)3 impregnation after pyrolysis at 700 degrees C and iron- modified hydrochar (Fe@RHC) with Fe(NO3)3 co-hydrothermal carbonization at 220 degrees C were prepared from rice husk raw, respectively. Soil incubation experiments were conducted by different environments or soil treatments, including air, pure oxygen, anaerobic, and sterilization, to approximate the soil nitrification, denitrification, and abiotic processes, aiming to investigate the contributions of iron-modified hydrochar on N2O emission pathways in rice soil compared with pyrochar, as well as the potential microbial regulatory mechanisms involved. The results showed that Fe@RHC reduced N2O emissions by 19.25 %, whereas 700Fe@RC significantly increased N2O release by 130.70 %. Denitrification was the dominant process driving N2O emissions following the application of iron-modified biochar in soil. Fe@RHC effectively promoted complete denitrification by optimizing the soil physicochemical environment (e.g., creating reducing conditions), enhancing the electron shuttle capacity within the soil, and increasing the abundance of denitrifying microbial genes, thereby reducing N2O emissions through the conversion of N2O to N2. In contrast, 700Fe@RC significantly increased the proportion of fungal denitrification, inhibited the activity of N2O reductase enzymes, and potentially introduced additional nitrate substrates, resulting in increased N2O emissions.