Enhancement of aerobic nitrogenase activity (acetylene reduction assay) by phenol in soils and the rhizosphere of cereals

By addition of phenol at concentrations between 0.1 and 10 mmol·l^?1, nitrogenase activity (acetylene reduction assay) is enhanced by a factor of 5 in the rhizosphere of Pennisetum glaucum (pearl millet) incubated under 20% O₂. No increase is found under microaerobic conditions. This enhancement effect is also noticed in a soil amended with a sucrose concentration of 20 mmol·l^?1. Under those conditions, however, an enhancement is found under aerobic as well as under microaerobic conditions and a further increase of the phenol added reduces the activity to almost zero. A 4-fold increase of N₂-fixation by phenol addition under aerobic conditions was determined with homogenous sediments from a fresh water lake while anaerobic N₂-fixation was already slightly reduced by the same concentration added. Excised roots of Sorghum nutans CSH 5 failed to show any phenol enhancement of nitrogenase activity. After a preincubation of 6h, inhibition of nitrogenase activity under air by addition of 1 mmol·l^?1 was much more pronounced than under microaerobic conditions.