The Role of Anaerobiosis in Asymbiotic Nitrogen Fixation

Microbial fixation of atmospheric nitrogen was measured in soil amended with ground wheat straw and incubated at two moisture levels (field capacity and waterlogged). Fixation equivalent to 42–52 kg/ha in the soil at field capacity and 13–150 kg/ha in the waterlogged soil was observed using 15 N techniques when the soil was amended with 1% straw or less. Using both 15 N and Kjeldahl techniques, high rates of fixation (500–1000 kg/ha) were measured in soils amended with 5–20% straw and incubated under waterlogged conditions.The high levels of nitrogen fixation in this study can be attributed to a combination of (a) aerobic conditions required to break down the plant residues and (b) anaerobic sites which are essential for significant fixation rates. The Clostridia are strongly implicated in this process in nature. Upon incubation in the laboratory, the nitrogen-fixing clostridia increased 1000-fold in the water-saturated soil and 100-fold in the field capacity soil. These organisms were also active in field soils throughout the growing season. Identification studies indicated that these organisms were primarily Clostridium butyricum.