Objective  The purpose of this study is to reveal the effects of combined application of biochar and biogas slurry on soil greenhouse gas emission in a poplar plantation system.

  Method  The greenhouse gas emission fluxes of different biochar and biogas slurry addition treatments were measured once a month from July 2019 to January 2020 based on the long-term field observation experiment station. The different treatments include four biogas slurry application rates (C, L, M, H) and three biochar application rates (B0, B1, B2). Soil physicochemical properties were measured in September 2019 and January 2020. The effects of biochar and biogas slurry on soil greenhouse gas emission were further revealed by statistical analysis.

  Result  Biochar application significantly increased soil total carbon (TC) (B0: 1.83 ± 0.14%, B1: 2.42 ± 0.14%, B2: 2.75 ± 0.14%) and total nitrogen (TN) (B0: 0.18 ± 0.01%, B1: 0.22 ± 0.01%, B2: 0.24 ± 0.01%). The biogas slurry application significantly increased soil TN, but there was no significant reciprocal action between biochar and biogas slurry on soil TC and TN. In winter, both biogas slurry and biochar significantly affected the soil ammonia concentrations, and there was a significant reciprocal action between them. The biogas slurry significantly increased the soil extractable nitrate concentrations. During the field monitoring period, biogas slurry significantly increased the cumulative emissions of soil N₂O and CH₄, and enhanced the total global warming potential. The total global warming potential for low (L), medium (M) and high (H) biogas slurry application increased by 30%, 40% and 44%, respectively, compared with the treatment without slurry application. In contrast, biochar application showed no significant effect on the cumulative greenhouse gases emissions and the total global warming potential.

  Conclusion  The inhibition effect of single biochar application on greenhouse gas emission is not significant after 7 years, and this application method may even increase soil greenhouse gas emission. The application of biogas slurry three times a year significantly promotes soil greenhouse gas emission. Therefore, it is recommended to further study the potential mechanism of aging effect on biochar and the optimal ratio of biochar and biogas slurry (nitrogen fertilizer) to improve soil environment and reduce greenhouse gases emission.