Algal and bacterial non-symbiotic nitrogen fixation in paddy soils

Summary The nitrogen fixing activity of three Ivory Coast soils was tested in the laboratory by the acetylene reduction assay and the Kjeldahl method. Nitrogen fixation due to algae was estimated to be of the order of 4 to 8 (acetylene method) and 7 μg N per g soil per day (Kjeldahl method). Nitrogen fixation due to bacterial activity in the rice rhizosphere was estimated to be of the order of 2 to 5 (acetylene method) and 1 to 3 μg N per g soil per day (Kjeldahl method). These results emphasize the importance of the bacterial nitrogen fixation in the rhizosphere which had been hitherto overlooked. Comparison of acetylene method and Kjeldahl method results shows discrepancies the origin of which has been discussed. Time course of acetylene reduction by rhizosphere soils exhibits a lag phase which may be attributed to Postgate's switch off — switch on process.