The organisms and biological processes involved in asymbiotic nitrogen fixation in waterlogged soil amended with straw.

The acetylene reduction assay was used to determine nitrogenase activity in a waterlogged soil-straw system. Nitrogenase activity was related to the number of nitrogen-fixing clostridia. The patterns of respiration, microbial numbers, and nitrogenase activity indicated a change in the microbial activity at about the 8th day of incubation. This was attributed to the sequential use of the different straw components, with a short lag in activity between the periods of maximum hemicellulose and cellulose utilization. Two layers of microbial activity were observed in waterlogged soil-straw mixtures, an aerobic layer extending from the surface to a depth of 2 mm, and an anaerobic layer below this. The organisms in the aerobic layer were responsible for the partial degradation of straw and the release of metabolites; the metabolites were used by the nitrogen-fixing organisms in the anaerobic layer. Anaerobic organisms of the genus Clostridi~rn were found to be the only active nitrogen fixers in the waterlogged systems. The physiological and morphological characteristics of the organisms isolated from the waterlogged soil are similar to those of C . butylicurn.