Antagonistic interaction between biochar and nitrogen addition on soil greenhouse gas fluxes: A global synthesis

Both biochar and nitrogen (N) addition have been proposed for enhancing plant productivity and increasing carbon (C) sequestration. Although numerous studies have been conducted to examine responses of soil greenhouse gas (GHG) fluxes to biochar or N addition, biochar is often co-applied with N fertilizer and the interactive effects of the two factors still remain unclear. In this study, we performed a meta-analysis of manipulative experiments with 267 two-factor observations to quantify the main and interactive effects of biochar and N addition on soil GHG fluxes at a global scale. Our results showed that biochar addition significantly increased soil CO₂ emission by 10.1%, but decreased N₂O emission by 14.7%. Meanwhile, N addition increased both soil CO₂ and N₂O emissions by 11.6% and 288%, respectively. The combination of biochar and N addition also exhibited significant positive effect on CO₂ (+18.0%) and N₂O (+148%) emissions, but there were non-significant changes in CH₄ fluxes. Consequently, antagonistic interaction between biochar and N addition was observed in soil GHG fluxes and their global warming potential (GWP), except for CH₄ uptake showing an additive interaction. This synthesis highlights the importance of the interactive effects between biochar and N addition, providing a quantitative basis to develop sustainable strategies toward widespread application of biochar to preserve cropping system and mitigate climate change.