Studies on the Effect of Aeration on Nitrate Reduction by Pseudomonas Species Using N ¹⁵

The obvious economic importance of losses of nitrogen from the soil by the action of denitrify-ing bacteria has stimulated investigations on factors affecting this process. That the partial pressure of oxygen influences this biological trans-formation in certain species has long been known. Various results of increasing the oxygen tension on nitrate reduction and denitrification have been reported; some of the more significant con-tributions have been summarized by Sacks and Barker (1949). These workers concluded that in Pseudomonas denitrificans "at oxygen tensions of about 5 per cent nitrate can be reduced only as far as nitiite and at partial pressures of oxygen in excess of 5 per cent denitrification failed to occur. " They attributed the reduction of nitrate and denitrification observed by other workers to variation in methods of maintaining "aerobic" conditions. In connection with a study on the utilization of nitrogen fertilizers by plants, nitrogen losses were observed although normal conditions of soil aeration had been obtained (MacVicar et al., 1951). This stimulated our interest in the rela-tionship between denitrification and oxygen supply. Careful survey of the literature revealed several studies in which nitrate reduction and denitrification had been reported despite what would appear to be the maintenance of adequate oxygen supply. Investigations were undertaken therefore using the stable isotope of nitrogen in the belief that the use of this sensitive technique might provide additional insight into this com-plex biochemical transformation.