Effects of Straw Return and Soil Amendments on Methane Emissions During Early Anaerobic Incubation of Paddy Soil

Abstract

Rice paddies are one of the largest global sources of anthropogenic methane (CH₄), and straw return can intensify emissions by supplying decomposable carbon and promoting reducing conditions. However, amendment-specific responses to rice straw-induced CH₄ emissions under anaerobic conditions remain insufficiently understood. A 15-day laboratory incubation experiment was conducted using 6 treatments: control (CK), straw only (S), straw combined with biochar (S+Bio), zeolite (S+Zo), CuSO₄ (S+Cu), and FeCl₃ (S+Fe), with three replicates. Flooded paddy soil was incubated at 25°C, and CH₄ flux, soil pH, electrical conductivity (EC), oxidation-reduction potential (ORP), dissolved H₂, and temperature were measured on days 0, 7, and 15. Results showed that straw return increased CH₄ emissions compared to CK, whereas amendment effects changed markedly over time. On day 7, S+Fe reduced CH₄ flux by 91.9% relative to S, the strongest early-stage suppression, but reversed by day 15 to the highest CH₄ flux among amended treatments. S+Bio and S+Zeo instead showed greater suppression at day 15 than at day 7, while S+Cu achieved the highest average suppression (45.2%) across the 15-day incubation. CH₄ was negatively correlated with ORP (r=-0.63, p<0.001) and positively correlated with dissolved H₂ (r=0.69, p<0.001), confirming the central role of soil redox status in CH₄ production. These findings demonstrate that amendment effectiveness during post-harvest straw decomposition is strongly time-dependent, highlighting the need to evaluate mitigation performance across the full decomposition period rather than at a single time point.