Effects of elevated CO2 and temperature on methane production and emission from submerged soil microcosm

Incubation experiments were conducted under controlled laboratory conditions to study the interactive effects of elevated carbon dioxide (CO2) and temperature on the production and emission of methane (CH4) from a submerged rice soil microcosm. Soil samples (unamended soil; soil + straw; soil + straw + N fertilizer) were placed in four growth chambers specifically designed for a combination of two levels of temperature (25 degreesC or 35 degreesC) and two levels of CO2 concentration (400 or 800 mu mol mol(-1)) with light intensity of about 3000 Lx for 16 h d(-1). At 7, 15, 30, and 45 d after incubation, CH4 flux, CH4 dissolved in floodwater, subsurface soil-entrapped CH4, and CH4 production potential of the subsurface soil were determined. The results are summarized as follows: 1) The amendment with rice straw led to a severalfold increase in CH4 emission rates, especially at 35 degreesC. However, the CH4 flux tended to decrease considerably after 15 d of incubation under elevated CO2. 2) The amount of entrapped CH4 in subsurface soil and the CH4 production potential of the subsurface soil were appreciably larger in the soil samples incubated under elevated CO2 and temperature during the early incubation period. However, after 15 d, they were similar in the soil samples incubated under elevated or ambient CO2 levels. These results clearly indicated that elevated CO2 and temperature accelerated CH4 formation by the addition of rice straw, while elevated CO2 reduced CH4 emission at both temperatures.