Biochar-derived persistent free radicals and reactive oxygen species reduce the potential of biochar to mitigate soil N₂O emissions by inhibiting nosZ

Abstract

Biochar amendment has been proven to generally reduce soil nitrous oxide (N₂O) emissions. However, the effects and underlying mechanisms of biochar-derived reactive oxygen species (ROS) on soil N₂O emissions are still unclear. Thus, we firstly weakened the intensities of persistent free radicals (PFRs) within biochar using triethanolamine (TEA) as a quencher, and then used soil incubation methods to compare the potentials of TEA-quenched and un-quenched biochar in mitigating soil N₂O emissions. The TEA-quenched biochar generated less hydrogen peroxide (H₂O₂) and hydroxyl radical (•OH), while having a higher soil N₂O emission mitigation potential, than the un-quenched biochar. The N₂O emissions and the N₂O/(N₂O + N₂) ratio were significantly, positively, correlated with the generated H₂O₂ and •OH contents. These results demonstrated that biochar-derived ROS weakened biochar's mitigation of soil N₂O emissions. The specific mechanisms of biochar-derived ROS on soil N₂O emissions were further explored by a ROS scavenging experiment. It was found that scavenging H₂O₂ by catalase efficiently hindered the generation of •OH, resulting in decreases in N₂O emissions and the N₂O/(N₂O + N₂) ratio. Meanwhile, biochar-derived ROS exhibited a more severe inhibition on N₂O reductase gene (nosZ) expression than that on the expression of genes responsible for N₂O production (nirK and nirS), indicating that biochar-derived ROS weakened biochar's mitigation of soil N₂O emissions by inhibiting microbial N₂O reduction. Our results imply that controlling the content of biochar-derived ROS is a promising strategy to maximize biochar's potential for mitigating soil N₂O emission.