Some Factors Affecting Asymbiotic Nitrogen Fixation in Soils as Measured by 15N Enrichment¹

Abstract

The use of the isotopic 15N technique to measure asymbiotic N fixation in soils has been reported by very few workers, despite the obvious advantages of the technique in assessing the importance of this fixation in agricultural soils. The effect of glucose plus cellulose (0 and 0.5% glucose + 0.5% cellulose), glucose alone (0, 0.04, 0.2, 1.0 and 5.0% on dry soil weight basis), incubation temperature, method of soil aeration, and atmospheric composition on N fixation in soil in a closed system using 15N was investigated. Significant fixation was observed mainly when a readily available supply of energy such as glucose plus cellulose or glucose alone was added to the soil. Aerating the energy‐enriched soil by diffusion resulted in greater N fixation under an aerobic and anaerobic atmosphere than aerating by a continuous air flow in a closed system. The optimum glucose concentration was 1% at the 15 and 35 C incubation temperatures employed, while 5% glucose resulted in greater fixation at the 25 C temperature. Generally, more N was fixed at 15 and 25 C than at the 35 C incubation temperature.

Evaluation of nitrogen fixation by bacteria in association with roots of tropical grasses.

“…It has been suggested that nitrogen fixation by free-living bacteria in cultivated land is widespread but generally at low rates (175). Measurements of acetylene reduction or incorporation of "'N2 into carbohydrateor cellulose-amended soils have demonstrated potentials for nitrogen fixation, which could occur periodically in nature and account for appreciable gains in nitrogen (18,37,49,69,75,93,122,124,151,165,185).…”