Fermentation pattern of methanogenic degradation of rice straw in anoxic paddy soil

The anaerobic degradation of different fractions of rice straw in anoxic pa ddy soil was investigated. Rice straw was divided up into stem, leaf sheath and leaf blade. The different straw fractions were mixed with paddy soil a nd incubated under anoxic conditions. Fermentation of straw components star ted immediately and resulted in transient accumulation of acetate, propiona te, butyrate, isobutyrate, valerate, isovalerate and caproate with much hig her concentrations in the presence than in the absence of straw. Also some unidentified compounds with UV absorption could be detected. The maximum co ncentrations of these compounds were different when using different straw f ractions, suggesting differences in the degradation pathway of these straw fractions during the early phase of incubation. i.e. with Fe(III) and sulfa te serving as oxidants. When concentrations of the intermediates decreased to background values, CH4 production started. Rates of CH4 production were higher in incubations with straw from the stem>leaf blade>leaf sheath>uname nded soil. During the methanogenic phase. the percentage contribution of Fe rmentation products to CH4 production was determined by inhibition with 2-b romoethanesulfonate (BES). Acetate (48-83%) and propionate (18-28%) were fo und to be the main intermediates of the carbon flow to CH4, irrespective of the fraction of the rice straw or its absence. Mass balance calculations s howed that 84-89% of CH4 was formed via acetate in the various incubations. Radiotracer experiments showed that 11-27% of CH4 was formed from H-2/CO2, thus confirming that acetate contributed 73-89% to methanogenesis. Our res ults show that the addition of rice straw and the fraction of the straw aff ected the fermentation pattern only in the early phase of degradation, but had no effect on the degradation pathway during the later methanogenic phas e. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.