Methanogenic responses to exogenous substrates in anaerobic rice soils

Soils collected from rice fields in the Philippines differed in their inherent potential for methane in vitro production and were tested for their response to organic amendments. Two soils were amended with either acetate or glucose (experiment I), root exudates (experiment II), and three soils were amended with rice straw (experiment III). Addition of acetate, glucose, or root exudates stimulated CH4 production in soil with high production capacity (Pangil) (16.0 μmol CH4 g−1) as well as low production capacity (Maahas) (0.171 μmol CH4 g−1). However, the response triggered by a given amendment was more pronounced in Pangil soil than in Maahas soil. Similarly, application of rice straw triggered the fastest response in the soil with high inherent production potential (Pila) (peaking at 2 weeks after incubation at 25°C) as compared to the soil with moderate (Luisiana) (peaking at 3 weeks) and low production potential (Maahas) (peaking at 4 weeks). In all experiments, soils with an inherently high production (Pangil, Pila) showed a faster and higher response than those with suppressed production (Luisiana, Maahas). The net increments of production rates were used to calculate the transformation efficiencies, i.e. the stochiometric rate of CH4 produced from a given substrate amendment. The transformation efficiencies of added substrates decreased in the order of glucose > acetate > root exudates > straw. High transformation efficiencies of acetate, glucose and root exudates indicated a priming effect, i.e. enhanced decomposition of soil organic matter through added substrate. This priming effect due to the reactivation of fermentative microflora by adding substrate C may also increase the effects of root exudates under field conditions.