Methane production capacities of different rice soils derived from inherent and exogenous substrates

Methane production rates were determined at weekly intervals during anaerobic incubation of eleven Philippine rice soils. The average production rates at 25 degrees C varied in a large range from 0.03 to 13.6 mu g CH(4) g((d.w.soil))(-1)d(-1). The development of methane production rates derived from inherent substrate allowed a grouping of soils in three classes: those with instantaneous development, those with a delay of approximately two weeks, and those with a suppression of methane production of more than eight weeks. Incubation at 30 and 35 degrees C increased production capacities of all soils, but the grouping of soils was still maintained. The Arrhenius equation provided a good fit for temperature effects on methane production capacities except for those sails with suppressed production. Acetate amendment strongly enhanced methane production rates and disintegrated the grouping. However, the efficiencies in converting acetate to methane differed among soils. Depending on the soil, 16.5-66.7% of the added acetate was utilized within five weeks incubation at 25 degrees C. Correlation analyses of methane production (over eight weeks) and physico-chemical soil parameters yielded significant correlations for the concentrations of organic carbon (R(2) = 0.42) and organic nitrogen (R(2) = 0.52). Correlation indices could substantially be enhanced by using the enriched fraction of organic carbon (R(2) = 0.94) and organic nitrogen (R(2) = 0.77), i.e. the differential between topsoil and subsoil concentrations of the respective compounds. The enriched organic material in the topsoil corresponds to the biologically active fraction and thus represents a good indicator of methane production derived from inherent substrate. The best indicators of the conversion rate of acetate in different soils were pH-value (R(2) = 0.56) and organic carbon content (R(2) = 0.52). Apparently, soil properties affect methane production through various pathways. Inherent organic substrate represents a considerable source of methane in some soils and is negligible in others. Likewise, soils also differ regarding the response to exogenous substrate. Both mechanisms yield in a distinct spatial variability of methane production in rice soils.