The effects and underlying mechanisms of modified biochar combined with nitrification inhibitors on nitrous oxide mitigation in acidic soils.

Acidic soils are recognized as hotspot for nitrous oxide (NO) emissions, and effective NO mitigation strategies are lacking. Biochar and its modified products have emerged as promising soil amendments for improving soil quality and mitigating NO emissions. However, the mitigation efficiency of biochar and modified biochar and whether it can synergize with the nitrification inhibitor (3,4-dimethylpyrazole phosphate; DMPP) to further decrease NO emission in acidic soils has not been sufficiently investigated. Therefore, this study investigated the effects and mechanisms of pristine biochar (B1), alkali-modified biochar (B2), and polyethylenimine-alkali-modified biochar (B3), applied alone or in combination with DMPP on N₂O emissions. The results showed that B2 application significantly decreased NO emission by 38.9 %, achieving higher mitigation efficiency than B1. Nevertheless, B3 application significantly increased NO emissions by 119.5 %. Combining B1 with DMPP did not achieve higher NO mitigation efficiency compared with DMPP alone, likely because B1 amendment observably improved soil organic carbon. Notably, a synergistic effect on N₂O mitigation (59.6 %) was observed when B2 was combined with DMPP. After alkali modification, B2, with the largest specific surface areas, recruited more NO reducing microbes and efficiently increased the nosZ gene abundance. In addition, B2 was prepared through high-temperature carbonization, resulting in lower C content, which did not limit the DMPP efficiency. DMPP alone significantly decreased NO emission by reducing the abundance of the amoA gene of ammonia-oxidizing bacteria. Therefore, the highest mitigation efficiency of B2 combined with DMPP is attributed to lower NO production through nitrification and enhancement of NO reduction via denitrification. Moreover, the improvement status of biochar on soil pH and organic carbon might determine whether the combination of biochar and DMPP has a synergistic or antagonistic effect on NO mitigation.