Biochar reduces nitrous oxide but increases methane emissions in batch wetland mesocosms

Biochar was added into constructed wetlands (CWs) as an amendment to the main substrate (i.e., coarse gravel) for improving the removal efficiency of pollutants and mitigating greenhouse gas (GHG) emissions. Four types of mesocosm-scale CWs, i.e., unamended subsurface batch CWs (SSBCWs) and surface batch CWs (SBCWs), and biochar-amended SSBCWs and SBCWs, were established in this study. The SSBCWs outperformed SBCWs in both removing pollutants (particularly COD, NO3--N and TN) and reducing the global warming potential (GWP), irrespective of adding biochar or not. The amendment of biochar improved the efficacy of CWs for removing pollutants and mitigating GHG emissions in both configurations of CWs. The highest removal percentages of COD (89.6%), NO3- -N (89.2%) and TN (92.5%) were obtained in biochar-amended SSBCWs, followed by unamended SSBCWs, biochar-amended SBCWs, and unamended SBCWs. The lowest GWP (5.252 mg/m(2)/h) was simultaneously obtained in biochar-amended SSBCWs, and the addition of biochar reduced GWP by 57.3% and 3.0% for SSBCWs and SBCWs, respectively. The abatement of GHG by biochar addition was mainly reflected in reduction of N2O fluxes, while the CH4 fluxes were promoted and the CO2 fluxes were not affected. The quantitative PCR results indicate that the reduced N2O fluxes in biochar-amended CWs were driven by the enhanced transcription of the nosZ gene and the ratio of nosZ/(nirS + nirK). This study demonstrates that biochar-amended SSBCWs can be an ideal alternative for design and application of CWs for removing pollutants and abating GHG emissions in the future.