Biochar mitigates the peatland GHG dilemma under contrasting water table regimes: phase-dependent responses of CO2 and CH4 over a two-year study

Peat soils are major terrestrial carbon stores, yet drainage alters redox conditions that stimulate CO 2 , CH 4 , and N 2 O 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 (WTL 20 ) provided the optimal balance between carbon loss and CH 4 suppression compared to saturated (WTL 0 ). Although CO 2 emissions increased under WTL 20 , CH 4 fluxes declined by over 90% relative to WTL 0 , where methanogenesis dominated. This shift in the Control (from WTL 0 toWTL 20 ), reduced overall CO 2 -equivalent emissions by 17 t CO 2 eq ha −1 yr −1 , highlighting the critical GHG balance of maintaining a WTL 20 in lowland peatlands. Among the amendments, labile, low C:N amendments (cereal straw, biosolids) increased CO 2 and N 2 O emissions under WTL 20 , indicating enhanced aerobic mineralization following oxygen exposure. In contrast, biochar consistently outperformed all treatments, reducing cumulative CO 2 emissions by up to 52% compared with the Control-WTL 40 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 (WTL 0 and WTL 20 ) in two consecutive years. Overall, this study demonstrates that integrating WTL 0 and WTL 20 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.