Quantifying biochar-induced greenhouse gases emission reduction effects in constructed wetlands and its...

Zero-waste biochar is an emerging tool for carbon neutralization, but the role of biochar in reducing greenhouse gases (GHGs) emissions from CWs were controversy and uncertainty. Yet, no previous study has integrated multiple research systems to quantitatively examine biochar-mediated GHGs emission reduction potential in CWs. Here we synthesized 114 studies to quantify biochar-induced declines ability of GHGs in the CWs by using the multi-level meta-analysis, reveal the variation of GHGs emission effect in different biochar-CWs and its response relationship with biochar, and identify the moderating variables that had a strong explanatory effect on the emission reduction effect of biochar. We showed that biochar remarkably affect CO₂ mitigation (p < 0.05), but has insignificant and heterogeneous effects on CH₄ and N₂O. Pyrolysis time, influent dissolved oxygen (DO), influent NO₃^--N concentration, hydraulic retention time (HRT) and wetland type can significantly affect the effect of biochar on CH₄ emission reduction. Particularly, the importance of HRT and wetland type was 0.89 and 0.85, respectively. Specially, the surface batch CWs modified by biochar could significantly promote the emission of CH₄ (p < 0.001), and the effect size was up to 89.59. For N₂O, biochar diameter, biochar addition ratio, influent COD/TN ratio, plant name, and removal efficiency of NO₃^--N/TN/COD were significant moderators. Among them, influent COD/TN ratio and plant name showed a stronger explanation. Planting Cyperus alternifolius L. significantly enhanced the N₂O emission reduction capacity by biochar (p < 0.001), and effect size was as low as -24.32. 700-900 °C biochar can promote CH₄ flux but inhibit N₂O flux. This study provides an important theoretical basis and valuable strategic guidance for more accurate estimation and improvement of synergistic emission reduction benefits between CH₄ and N₂O of biochar in CWs.