Nitrogen fixation by free-living heterotrophic bacteria in an oak forest: The effect of liming

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Abstract

Nitrogen-fixing heterotrophic bacteria, mainly Clostridium butyricum and, less frequently, Enterobacter agglomerons and Klebsiella pneumoniae, are found throughout the aerial and soil layers of an oak forest. However they are only active in nitrogen fixation in the soil. Soil slurry experiments show that the main factors limiting N2 fixation in the forest are low temperature, low pH and shortage of C sources. Raising the pH of the soil with lime to pH 6, which is the optimum for N2 fixation by C. butyricum in soil slurries, more than doubles the rate of N2 fixation (as measured by in situ15N2 methods) from 7.84 to 16.1 kg N ha−1yr−1. The N fixed by C. butyricum can rapidly be taken up by oak seedlings and translocated to the actively-growing leaves.

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