Combined Effects of Biochar and Dissolved Organic Matter Surrogate AQDS on Methane Emissions from Paddy Soil

• • Biochar amendment increased maximum cumulative CH4 emission to (66.23±16.20) μmol, a 4.4-fold increase over control (15.14±0.18) μmol, demonstrating its potential to enhance methanogenesis in paddy soils, which is critical for assessing biochar's net greenhouse gas impact. • • AQDS addition suppressed CH4 accumulation to (1.04±0.09) μmol, a 93% reduction relative to control, due to sulfate acting as a competitive electron acceptor; this highlights the risk of using sulfate-containing amendments to mitigate CH4. • • Combined biochar and AQDS resulted in intermediate CH4 accumulation of (12.71±0.32) μmol, which is 19% of biochar alone and 12-fold higher than AQDS alone, indicating an additive effect where biochar's stimulation is partially offset by AQDS-induced electron competition. • • The combined treatment favored the acetoclastic methanogenesis pathway, which produces less CH4 per unit acetate, explaining the lower CH4 yield compared to biochar alone; this mechanistic insight is essential for predicting CH4 emissions under co-amendment scenarios.