Effect of various anionic species on net methane production in flooded rice soils

In a laboratory incubation study, effect of various anions on net methane p roduction in two rice soils (alluvial and acid sulphate) under flooded cond itions was examined. Methane production was considerable in alluvial soil a nd almost negligible in acid sulphate soil, albeit with a higher density of viable methanogens, during 30-day incubation without salts. Sodium salts o f hydroxide and phosphate further stimulated methane production in alluvial soil and marginally in acid sulphate soil. But, addition of sodium molybda te, a selective inhibitor of sulphate-reducing bacteria, increased the prod uction of methane in acid sulphate soil. In contrast, nitrite, nitrate, sul phite and sulphate suppressed the production of methane in both soils. Acet ate served as an excellent substrate for methanogenesis in alluvial soil, b ut not in acid sulphate soil. Succinate and citrate also stimulated methane production especially in alluvial soil, but after a longer lag. In acid su lphate soil, most of the added carbon in the form of sodium salts of carbox ylic acids was converted to CO2 and not methane, which is consistent with t heir preferential use by the sulphate-reducing bacteria. In general, none o f the amendments could increase production of methane in acid sulphate soil to the same level as in alluvial soil.