Non-symbiotic nitrogen fixation in soil

Abstract

1. The fixation of N15-labeled nitrogen in small vessels of California soil under various conditions of pH, substrate level, oxygen tension, and other soil conditions was observed. 2. Nitrate concentrations greater than 1.0-1.5 microequivalents per gram soil were found to suppress nitrogen fixation but not the growth of Azotobacter. 3. Large amounts of nitrogen were fixed when soluble organic substrates (e.g. glucose or sucrose) were added to the soil. 4. Moderate fixation also resulted from the inversion of a disc of sod. 5. Fixed nitrogen appeared largely in the nitrate and ammonia-amide fractions with that in the nitrate fraction probably representing nitrification of more reduced initial products of fixation. 6. Under conditions of these experiments growing grass did not enhance fixation. At higher light intensities, however, such an enhancement might be observed. 7. The incorporation of grass cuttings, straw or alfalfa meal into the soil caused only a slight increase in fixation. 8. The inoculation of soils with large populations of Azotobacter did not result in increased fixation. © 1956 Martinus Nijhoff.