Methanogenic degradation of rice plant buried in paddy soil

We investigated the degradation of three fractions of matured rice plant (stem, leaf and root) in paddy soil. To determine the loss of weights of these fractions due to degradation, 0.5 g of each fraction were sealed in a nylon bag (mesh size 25μm) and buried in a glass bottle containing a mixture of 800 g soil and 260 ml water. The losses of dry weight of stem and leaf were 60% and 50%, respectively, after incubation at 30degC for 110 days. Root was the most refractory to degradation, showing only 25% loss of dry weight. For the determination of dehydrogenase activity and the production of CO2 and CH4 accompanying the degradation of polysaccharides in the plant, 1.5 g of each fraction were sealed in a glass bottle with soil-water mixture of 400 g soil and 130 ml water. The bottles were incubated at 30degC for 30 days without shaking. The highest microbial activity was observed in the bottle containing the stem, in which the microbial population reached the stationary state at the 11th day during the incubation period. In the bottle containing leaf, the stationary state of the population was realized at the 25th day during incubation. In both bottles, CH4 formation reached the maximum at the 11th day and then decreased. In the bottle containing root, though microbial activity slowly kept increasing throughout the incubation period, CH4 formation from root was not observed. The production of CO2 was observed prior to CH4 formation in all bottles. (author)