This study aims to comprehend the links between soil Eh and pH changes in eucalyptus biochar (BC) and rice straw (RS) amended soils and CH4 emissions. Increased CH4 emission rates and high total CH4 emissions (TCH4 ) were found in RS soils. In contrast, higher concentrations of refractory lignin, fixed C and volatile matter in BC suppressed C mineralization and terminal methanogenesis, resulting in low TCH4 . Eh in RS soils decreased more rapidly than in BC soils during the first phase as a single exponential function. This indicated that RS is a fast electron donor for an instant electron acceptor reduction and methanogenesis. During the second phase, Eh in BC soils decreased to very low values, probably because of the higher concentrations of electron donating phenolic compounds coupled with terminal methanogenesis. Meanwhile, hydrogen is consumed via electron acceptor reduction and methanogenesis simultaneously produce OH- corresponded with a rise of pH, a characteristic of reverse single exponential function.
