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

Abstract

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 methanogenesis. Therefore, the anaerobic turnover of [U- 14C]cellulose was investigated in fresh, non-methanogenic and in preincubated, methanogenic slurries of Italian rice field soil. In anoxic soil slurries freshly prepared from air-dried soil [U- 14C]cellulose was converted to 14CO 2 and 14CH 4 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- 14C]cellulose was converted to 14CO 2 and 14CH 4 in the ratio of 1:1. The turnover times (7–14 days) of cellulose degradation were not significantly different ( P>0.05) in fresh and methanogenic soil. Chloroform addition abolished CH 4 production, but only slightly (30%) inhibited cellulose degradation in both fresh and methanogenic soil. Under both soil conditions, [ 14C]acetate was the only labeled intermediate detected. A maximum of 24% of the applied radioactivity was transiently accumulated as [ 14C]acetate in both fresh and methanogenic soil slurries. However, when methanogenesis was inhibited by chloroform, 46% and 66% of the applied radioactivity were recovered as [ 14C]acetate in fresh and methanogenic soil, respectively. Only non-radioactive propionate accumulated during the incubation with [U- 14C]cellulose, especially in the presence of chloroform, indicating that propionate was produced from substrates other than cellulose.