Size-dependent fractions released from biochar govern the mechanism of spatiotemporal ROS evolution in charosphere: influence on nitrous oxide emissions

Reactive oxygen species (ROS) hotspots formation is initiated by the redox fluctuation in soil, primarily arising from the input of exogenous organic carbon. However, how different mobile fractions derived from biochar regulate ROS generation in the charosphere soil (microzone surrounding biochar) remains poorly understood. Our study found that, for biochar pyrolyzed at 300 °C, the < 0.45 µm (dissolved black carbon, DBC) fractions released from biochar dominated ROS generation in the charosphere via the indirect pathway. This can be attributed to their higher content of water-solublephenols, which increased Fe(II) accumulation by 68.4% and thereby promoted the Fenton reaction. However, for biochar pyrolyzed at 500 °C, the main contributors to ROS generation were the released 0.45–25 µm fractions (minute particles). These minute particles not only directly generate ROS due to their abundant persistent free radicals but also indirectly generate ROS by enriching biotic factors (e.g., Bacillus, Mycoarthris) via secreting redox-active metabolites. These distinct mechanisms underlie the different spatiotemporal dynamics of ROS hotspots in the charosphere. Specifically, the O2•- and H2O2 contents in the charosphere of biochar pyrolyzed at 500 °C were 1.69 and 1.29 times higher than those at 300 °C, while the •OH content was 1.23 times higher at 300 °C than at 500 °C. Furthermore, the resulting ROS, especially H2O2, weakened the ability of biochar to mitigate N2O emissions by 22.5% and 27.6% in the charosphere of biochar pyrolyzed at 300 °C and 500 °C, respectively. Our findings highlight that the size-dependent fractions released from biochar regulate the underlying mechanisms of ROS generation, offering novel insights into the potential of the charosphere soil in mitigating greenhouse gas emissions.