Loading of redox-active metal Fe largely enhances the capacity of biochar to mitigate soil N2O emissions by promoting complete denitrification

Nitrous oxide (N₂O) is a critical greenhouse gas and an ozone-depleting substance, with a global warming potential 298–310 times greater than that of CO₂. Mitigating N₂O emissions from soils has environmental benefits. Recent research indicates that biochar can serve as an “electron shuttle” to reduce N₂O emissions from soils. Electron shuttle is defined as organic molecules capable of reversibly receiving and donating electrons. Thus, biochar is expected to facilitate stepwise reduction of denitrification products, reducing N₂O to environmentally harmless N₂. However, it remains uncertain whether biochar’s capacity to mitigate N₂O can be enlarged by augmenting its function as an electron shuttle. Thus, this study prepared a biochar with enhanced electron shuttle potential by loading redox-active (Fe) onto biochar. The effectiveness of this biochar in mitigating soil N₂O emissions was investigated by incorporating it into the soil. The results showed that Fe-loaded biochar significantly augmented its function as an electron shuttle and dramatically reduced soil N₂O emissions by 92% compared to the original biochar. The degree of decrease in N₂O emissions was strongly associated with both the electron shuttle capacity and the concentration of redox-active Fe in the biochar. Additionally, Fe-loaded biochar significantly decreased the N₂O/(N₂O + N₂) emission ratio and increased the expression of the nosZ-II gene. Our findings suggest that redox-active Fe loading in biochar is an effective strategy to enhance its electron shuttle function. The augmented electron shuttle function of biochar can successfully facilitate N₂O mitigation emission by promoting complete denitrification.