Methanogenic responses to exogenous substrates in anaerobic rice soils

Soils collected from rice fields in the Philippines differed in their inher ent potential for methane in vitro production and were tested for their res ponse to organic amendments.. Two soils were amended with either acetate or glucose (experiment I), root exudates (experiment II), and three soils wer e amended with rice straw (experiment III). Addition of acetate, glucose, o r root exudates stimulated CH4 production in soil with high production capa city (Pangil) (16.0 mu mol CH4 g(-1)) as well as low production capacity (M aahas) (0.171 mu mol CH4 g(-1)). However, the response triggered by a given amendment was more pronounced in Pangil soil than in Maahas soil. Similarl y, application of rice straw triggered the fastest response in the soil wit h high inherent production potential (Pila) (peaking at 2 weeks after incub ation at 25 degrees C) as compared to the soil with moderate (Luisiana) (pe aking at 3 weeks) and low production potential (Maahas) (peaking at 4 weeks ). In all experiments, soils with an inherently high production (Pangil, Pi la) showed a faster and higher response than those with suppressed producti on (Luisiana, Maahas). The net increments of production rates were used to calculate the transformation efficiencies, i.e. the stochiometric rate of C H4 produced from a given substrate amendment. The transformation efficienci es of added substrates decreased in the order of glucose > acetate > root e xudates > straw. High transformation efficiencies of acetate, glucose and r oot exudates indicated a priming effect, i.e. enhanced decomposition of soi l organic matter through added substrate. This priming effect due to the re activation of fermentative microflora by adding substrate C may also increa se the effects of root exudates under held conditions. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.