Soil Redox and pH Effects on Methane Production in a Flooded Rice Soil

Methane formation in soil is a microbiological process controlled by many factors. Of them soil redox potential (Eh) and soil pH are considered critical controls. A laboratory incubation experiment was conducted to study the critical initiation soil Eh, the optimum soil pH, and the interaction of Eh and pH on CH4 production. The critical soil Eh for initiation of CH4 production observed was approximately from −150 to −160 mV. Between -230 and −150 mV, the relationship of CH4 production and soil Eh appeared to be negatively exponential (Y = a10-bx, where Y is the CH4 production rate, µg g-1 d-1; x the soil Eh, mV; and a and b are constants). The optimum pH of CH4 production was near neutrality. A small decrease in pH resulting from the introduction of acidic materials significantly decreased CH4 production. A slight increase in soil pH (about 0.2 unit higher than the natural soil suspension pH), however, resulted in an enhancement of CH4 production by 11 to 20% and 24 to 25% at controlled Eh of -250 and -200 mV, respectively. Results suggest that a decrease in CH4 emissions could be obtained by a small reduction in soil pH in Crowley soil (fine, montmorillonitic, thermic Typic Albaqualf).