Methanogenic responses to exogenous substrates in anaerobic rice soils

Soils collected from rice ®elds in the Philippines di€ered 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 mmol CH4 g ÿ1) as well as low production capacity (Maahas) (0.171 mmol CH4 g ÿ1). 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 258C) 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 eciencies, i.e. the stochiometric rate of CH4 produced from a given substrate amendment. The transformation eciencies of added substrates decreased in the order of glucose > acetate > root exudates > straw. High transformation eciencies of acetate, glucose and root exudates indicated a priming e€ect, i.e. enhanced decomposition of soil organic matter through added substrate. This priming e€ect due to the reactivation of fermentative micro ̄ora by adding substrate C may also increase the e€ects of root exudates under ®eld conditions. 7 2000 Elsevier Science Ltd. All rights reserved.

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