Biochar's electron shuttle potential mitigates N2O emissions by counteracting the stimulatory effect of rice root iron plaque

Iron (Fe) plaque on rice roots can enhance nitrous oxide (N2O) emissions from paddy soil, primarily through Fe (II) oxidation-coupled denitrification. In this study, our hypothesis is that biochar will reduce N2O emissions via an electron shuttle and complete denitrification. To test this hypothesis, we performed laboratory microcosm experiments using a paddy soil-Fe plaque system amended with biochar. We examined the effects of biochar on soil N2O emissions, soil microbial community composition, and denitrifying functional gene. Furthermore, we evaluated potential correlations between biochar's electron shuttle capacity and N2O emissions, as well as the N2O/(N2O + N2) ratio derived from denitrification processes. Our results demonstrated that, in the absence of rice straw biochar, N2O emissions were doubled in the presence of Fe plaque. Interestingly, the addition of 1 % biochar to the paddy soil neutralized the difference in N2O emissions between the Fe plaque and control treatments. Furthermore, biochar addition enhanced the abundance of Fe(II)-oxidizing denitrifiers (e.g., Bacillus and Zoogloea) at the genus level and upregulated key denitrification functional genes (e.g., nirK and nosZ) associated with N2O mitigation. Importantly, oxidative treatment of biochar with H2O2 at varying concentrations reduced its electron donation capacity, which significantly weakened its ability to counteract Fe plaque-induced stimulation of N2O emissions. This efficacy was directly proportional to the biochar's electron transfer capabilities. These results highlight the critical role of biochar's electron transfer function in counteracting the stimulatory effect of Fe plaque on N2O emissions. We conclude that adding biochar with strong electron transfer capabilities is a promising strategy to curb the Fe plaque-induced priming effect on N2O emissions in paddy soils. The application of just 1 % biochar at the rice seedling stage may effectively mitigate N2O emissions in paddy soils.