A study was made of the N cycle in permanent pastures. Between 10 and 40 % was lost when labelled NO 3 -, was added. The losses by denitrification were stimulated by living roots, which decreased the O 2 level in the rhizosphere and excreted organic compounds which could act as hydrogen donors during NO 3 - reduction. Experiments with sterile plants demonstrated that Pseudomonas and Achromobacter species could use the root's excretory products as hydrogen donors. Bacillus licheniformis and some other B.spp., which could denitrify in pure culture, could not utilize these compounds. Amino acids stimulated denitrification by Pseudomonas aeruginosa and the rate-limiting reduction from nitrate to nitrite.B. licheniformis was not an obligate denitrifier; the presence of NO 3 - was only indispensable and a high nitrate reductase activity was only found if glycerol was the hydrogen donor.Conditions in the soil were not suitable for NO 3 - reduction to NH by B. licheniformis.
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