Biochar mitigates the peatland GHG dilemma under contrasting water table regimes: phase-dependent responses of CO 2 and CH 4 over a two-year study

Peat soils are major terrestrial carbon stores, yet drainage alters redox conditions that stimulate CO2, CH4, and N2O emissions. We conducted a two-year mesocosm experiment to quantify how water table level (0, 20, and 40 cm) interacts with organic amendments to regulate gaseous C and N fluxes from lowland peat. Amendments included Miscanthus biochar, Miscanthus chip, paper waste, biosolids, and cereal straw. Results revealed that moderate drainage (WTL20) provided the optimal balance between carbon loss and CH4 suppression compared to saturated (WTL0). Although CO2 emissions increased under WTL20, CH4 fluxes declined by over 90% relative to WTL0, where methanogenesis dominated. This shift in the Control (from WTL0 toWTL20), reduced overall CO2-equivalent emissions by 17 t CO2eq ha−1 yr−1, highlighting the critical GHG balance of maintaining a WTL20 in lowland peatlands. Among the amendments, labile, low C:N amendments (cereal straw, biosolids) increased CO2 and N2O emissions under WTL20, indicating enhanced aerobic mineralization following oxygen exposure. In contrast, biochar consistently outperformed all treatments, reducing cumulative CO2 emissions by up to 52% compared with the Control-WTL40 when assessed over the full 730-day experimental period. The consistent reduction in GHG emissions indicates constrained peat carbon mineralization under biochar amendment across contrasting hydrological conditions (WTL0 and WTL20) in two consecutive years. Overall, this study demonstrates that integrating WTL0 and WTL20 water table manipulations alternatively with stable, recalcitrant amendments such as biochar substantially altered greenhouse gas fluxes, offering a promising strategy to mitigate emissions while adding to and maintaining peat carbon stocks and fluxes from lowland bare peat. Graphical Abstract: