Nitrogen nonsymbiotic fixation

Second only to carbon inputs, nitrogen fixation by both symbiotic and nonsymbiotic organisms comprises an enormous biologically driven elemental influx to soils. Biological nitrogen fixation occurs via the following reaction (Allen et al., 1994) ... 

Utilize in nitrogen fixation. Until more data are available we can only guess as to whether nonsymbiotic bacteria or legume nodules are the main source of the apparently abundant nitrogen supply found in the Tropics. 

A limited number of comparatively recent experiments, using N, has indicated that the first products of fixation occur not in the bacteroids but rather in the plant tissues and cytoplasm immediately surrounding the bacteroids. Since fixation can occur only when bacteroid tissue is present and attached to the whole plant, it is obvious that Hxation is a truly symbiotic process. Much more biochemical work will be needed to work out the details of the process. The progress made in recent years in the study of the mechanism of nitrogen flxation in nonsymbiotic Hxation will doubtless help materially in the solution of the symbiotic fixation puzzle. 

The mechanism of nitrogen fixation is generally considered to be similar in both nonsymbiotic and symbiotic systems although most of the biochemical research has been with the former. The fixation process is undoubtedly a reductive process with little or no energy requirements, but the exact mechanism remains to be determined. Ammonia is considered a key compound in nitrogen fixation although not necessarily the first prod-... 

Attention has been drawn in recent years to the importance of molybdenum as a result of researches showing its essentiality in both symbiotic and nonsymbiotic nitrogen fixation. Many soils in Australia that formerly yielded only very poor crops of clovers and other legumes were found to yield abundantly when supplied with a few ounces of molybdenum per acre. The growth response is the result of greatly increased nitrogen... 

Review of litei ature on nonsymbiotic nitrogen fixation. Good bibliography included. 

Dommergues, Y., Balandreau, J., Rinaudo, G., Weinhard, P, 1973. Nonsymbiotic nitrogen fixation in the rhizosphere of rice, maize and different tropical grasses. Soil Biol. Biochem. 5, 83-89. 

---