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

Abstract

The potentials for sequential reduction of inorganic electron acceptors and production of methane have been examined in sixteen rice soils obtained from China, the Philippines, and Italy. Methane, CO₂, Fe(II), NO ⁻₃ , SO ²⁻₄ , pH, E_h, H₂ and acetate were monitored during anaerobic incubation at 30 °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 reduction phase during which most of the inorganic electron acceptors were depleted and CO₂ production was at its maximum, (2) a methanogenic phase during 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 reduction phases lasted for 19 to 75 days with maximum CO₂ production of 2.3 to 10.9μmol 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 3.08μmol d⁻¹ g⁻¹. During steady state the constant CH₄ and CO₂ production rates varied from 0.07 to 0.30μmol d⁻¹ g⁻¹ and 0.02 and 0.28μmol d⁻¹ g⁻¹, respectively. Within the 120 d of anaerobic incubation only 6–17% of the total soil organic carbon was released into the gas phase. The gaseous carbon released consisted of 61–100% CO₂, <0.1–35% CH₄, and <5% nonmethane hydrocarbons. Associated with the reduction of available Fe(III) most of the CO₂ was produced during the reduction phase. The electron transfer was balanced 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 produced (r =0.775) correlated best with the ratio of soil nitrogen to electron acceptors. Total nitrogen content was a better indicator for “available” organic substrates than the total organic carbon content. The redox potential was not a good predictor of potential CH₄ production. These observations indicate that the availability of degradable organic substrates mainly controls the CH₄ production in the absence of inorganic electron acceptors.