Biochar (BC) is a promising soil amendment for mitigating nitrous oxide (N2O) emissions. However, field experiments have reported inconsistencies in the changes in N2O emissions, and the underlying microbial mechanisms are unclear. A tobacco (Nicotiana tabacum L.) field under different tobacco BC application rates (0, 1, 10, 25, and 50 t ha(-1)) was established to investigate the changes of soil N2O emissions and microbial community compositions. BC amendments significantly increased the cumulative N2O emissions by 1.96-4.18 folds, mainly due to enhanced soil substrate availability under tobacco BC application. Shifts of bacterial community structure at the phylum level under BC amendment were observed, while changes in the structure of soil fungi at the genus level occurred. The abundance of denitrifying bacteria (Bradyrhizobium and Pseudomonas) and denitrifying fungi (Trichocladium and Trichoderma) was significantly increased with BC amendment, contributing to the stimulated soil N2O emissions by affecting aerobic denitrification. The field N2O mitigation of BC application should be reconsidered if tobacco BC is applied to upland soils.

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