Effect of soil characteristics on sequential reduction and methane production in sixteen rice paddy soils from China, the Philippines, and Italy

The potentials for sequential reduction of inorganic electron acceptors and production of methane have been examined in sixteen rice soils obtained fr om China, the Philippines, and Italy. Methane, CO2, Fe(II), NO3-, SO42-, pH , E-h, H-2 and acetate were monitored during anaerobic incubation at 30 deg rees C for 120 days. Based on the accumulation patterns of CO2 and CH4, the reduction process was divided into three distinct phases: (1) an initial r eduction phase during which most of the inorganic electron acceptors were d epleted and CO2 production was at its maximum, (2) a methanogenic phase dur ing which CH4 production was initiated and reached its highest rate, and (3 ) a steady state phase with constant production rates of CH4 and CO2. The r eduction phases lasted for 19 to 75 days with maximum CO2 production of 2.3 to 10.9 mu mol d(-1) g(-1) dry soil. Methane production started after 2 to 87 days and became constant after about 38-68 days (one soil > 120 days). The maximum CH4 production rates ranged between 0.01 and 3.08 mu mol d(-1) g(-1). During steady state the constant CH4 and CO2 production rates varied from 0.07 to 0.30 mu mol d(-1) g(-1) and 0.02 and 0.28 mu mol d(-1) g(-1), respectively. Within the 120 d of anaerobic incubation only 6-17% of the t otal soil organic carbon was released into the gas phase. The gaseous carbo n released consisted of 61-100% CO2, < 0.1-35% CH4, and < 5% nonmethane hyd rocarbons. Associated with the reduction of available Fe(III) most of the C O2 was produced during the reduction phase. The electron transfer was balan ced between total CO2 produced and both CH4 formed and Fe(III), sulfate and nitrate reduced. Maximum CH4 production rate (r = 0.891) and total CH4 pro duced (r = 0.775) correlated best with the ratio of soil nitrogen to electr on acceptors. Total nitrogen content was a better indicator for "available" organic substrates than the total organic carbon content. The redox potent ial was not a good predictor of potential CH4 production. These observation s indicate that the availability of degradable organic substrates mainly co ntrols the CH4 production in the absence of inorganic electron acceptors.