Straw-to-biochar shift in alternate wetting and drying paddies: Unlocking carbon sequestration and net GWP reduction synergies

Straw return and straw-derived biochar amendment are widely used to enhance soil carbon sequestration and fertility. However, few studies have compared their effects on soil carbon sequestration and net global warming potential (GWP) in paddy fields under equivalent straw inputs, especially in alternate wetting and drying irrigation (I AWD ). A 3-yr lysimeter experiment was therefore conducted with four treatments: continuous flooding irrigation (I CF ) with no amendment (I CF A 0 ), I AWD with no amendment (I AWD A 0 ), I AWD plus straw return (I AWD A S ), and I AWD plus biochar derived from equivalent straw (I AWD A B ). Results indicated that the shift from I CF to I AWD reduced CH 4 emissions by 9.14–30.17 kg ha⁻ 1 , and decreased total GWP by 0.11–0.57 t CO₂-eq ha⁻ 1 , but increased N 2 O emissions by 0.52–0.95 kg ha⁻ 1 . Compared to I AWD A 0 , I AWD A S significantly increased soil carbon sequestration (0~20 cm) by 9.5% and reduced N 2 O emissions by 25.8% over the three years; it also caused extraordinary increases in CH 4 emissions by 1792.0% over a 100-year GWP time horizon. In contrast, I AWD A B mitigated CH 4 and N 2 O emissions by 25.0% and 31.0%, respectively. In addition, I AWD A B reduced net GWP by nearly 338.9% compared to I AWD A S , primarily due to a 25.4% increase in soil carbon sequestration (29.07–36.46 t C ha −1 ) following the shift from straw to biochar. Life cycle assessment indicated that lower life-cycle greenhouse gas emissions under I AWD A B were driven by 7.33 t C ha −1 higher soil carbon sequestration, despite 0.44 kg ha −1 slightly higher production emissions than I AWD A S . Therefore, the straw-to-biochar shift represents a sustainable strategy to boost soil carbon sequestration and reduce net GWP in I AWD rice systems.