The role of anaerobiosis in asymbiotic nitrogen fixation.

niIicrobial fixation of atmospheric nitrogen was ~neasured in soil amended with ground wheat straw and incubated a t two moisture levels (field capacity and \vaterlogged). Fixation equivalent to 42-52 kg/ha in the soil a t held capacity and 13-150 I;g/ha in the waterlogged soil was observed using 15N techniques when the soil was amended with lyo straw or less. Using both I5N and ICjeldahl techniques, high rates of hsation (500-1000 kg/ha) were measured in soils amended with 5520% 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-hxing clostridia increased 1000-fold in the watersaturated 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 Clostridiz~vn butyriczlm.