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 (N₂O) emissions, and effective N₂O mitigation strategies are lacking. Biochar and its modified products have emerged as promising soil amendments for improving soil quality and mitigating N₂O 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 N₂O 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 N₂O emission by 38.9 %, achieving higher mitigation efficiency than B1. Nevertheless, B3 application significantly increased N₂O emissions by 119.5 %. Combining B1 with DMPP did not achieve higher N₂O 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 N₂O 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 N₂O 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 N₂O production through nitrification and enhancement of N₂O 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 N₂O mitigation.