Loading of redox-active metal Fe largely enhances the capacity of biochar to mitigate soil NO emissions by promoting complete denitrification

Nitrous oxide (NO) is a critical greenhouse gas and an ozone-depleting substance, with a global warming potential 298–310 times greater than that of CO. Mitigating NO emissions from soils has environmental benefits. Recent research indicates that biochar can serve as an “electron shuttle” to reduce NO 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 NO to environmentally harmless N. However, it remains uncertain whether biochar’s capacity to mitigate NO 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 NO 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 NO emissions by 92% compared to the original biochar. The degree of decrease in NO 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 NO/(NO + N) 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 NO mitigation emission by promoting complete denitrification.