Synergistically mitigating nitric oxide emission by co-applications of biochar and nitrification inhibitor in a tropical agricultural soil

Agricultural soils are the hotspots of nitric oxide (NO) emissions, which are related to atmospheric pollution and greenhouse effect. Biochar application has been recommended as an important countermeasure, however, its mitigation efficiency is limited as biochar, under certain conditions, can stimulate soil nitrification. Therefore, biochar co-applied with nitrification inhibitor could optimize the mitigation potential of biochar. Herein, a laboratory-scale experiment was conducted to investigate the effects of co-application of biochar and nitrification inhibitor on NO emission, nitrogen cycling function and bacterial community in a tropical vegetable soil. Results showed that a single application of biochar or nitrification inhibitor significantly decreased NO emissions, and this mitigation effectiveness was amplified by their co-applications. Soil NO₂⁻−N intensity, along with abundances of AOB-amoA and nirK were significantly and positively correlated with cumulative NO emissions. The stimulated activity of ammonia monooxygenase and growths of AOB and total comammox Nitrospira by biochar were weakened by nitrification inhibitor, implying decreased nitrification-driven NO production. The nitric oxide reductase activity and related qnorB abundance in nitrification inhibitor-added soils were increased by biochar, indicating promoted NO consumption during denitrification. The nirK abundance and NO₂⁻−N intensity were decreased more by co-applications of biochar or nitrification inhibitor. Moreover, both biochar and nitrification inhibitor changed bacterial β-diversity, and their co-application synergistically enriched Armatimonadetes and Verrucomicrobia abundances and decreased WPS-2 abundance. This study highlights that co-applications of biochar and nitrification inhibitor can make their respective advantages complementary to each other, thereby achieving a larger mitigation of NO emissions from agricultural soils in tropical regions.