Pattern of non-methanogenic and methanogenic degradation of cellulose in anoxic rice field soil

Rice field soils turn anoxic upon flooding. The complete mineralization of organic matter, e.g. cellulose, to gaseous products is then accomplished by the sequential reduction of nitrate, ferric iron, sulfate and finally by m ethanogenesis. Therefore, the anaerobic turnover of [U-C-14]cellulose was i nvestigated in fresh, non-methanogenic and in preincubated, methanogenic sl urries of Italian rice field soil. In anoxic soil slurries freshly prepared from air-dried soil [U-C-14]cellulose was converted to (CO2)-C-14 and (CH4 )-C-14 in a ratio of 3:1. In methanogenic soil slurries, on the other hand, which had been preincubated for 45 days under anaerobic conditions, [U-C-1 4]cellulose was converted to (CO2)-C-14 and (CH4)-C-14 in the ratio of 1:1. The turnover times (7-14 days) of cellulose degradation were not significa ntly different (P > 0.05) in fresh and methanogenic soil. Chloroform additi on abolished CH4 production, but only slightly (30%) inhibited cellulose de gradation in both fresh and methanogenic soil. Under both soil conditions, [C-14]acetate was the only labeled intermediate detected. A maximum of 24% of the applied radioactivity was transiently accumulated as [C-14]acetate i n both fresh and methanogenic soil slurries. However, when methanogenesis w as inhibited by chloroform, 46% and 66% of the applied radioactivity were r ecovered as [C-14]acetate in fresh and methanogenic soil, respectively. Onl y non-radioactive propionate accumulated during the incubation with [U-C-14 ]cellulose, especially in the presence of chloroform, indicating that propi onate was produced from substrates other than cellulose. (C) 2000 Federatio n of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.