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Out of Thin Air
Natural History, Sept, 2001 by David W. Wolfe
Each type of nitrogen-fixing bacterium must also devise a mechanism for shielding nitrogenase from oxygen, because exposure to oxygen destroys this all-important enzyme. Rhizobia on a pea plant, for example, signal the plant to synthesize leghemoglobin, a large molecule that has an even higher affinity for oxygen than does the hemoglobin of our blood. The leghemoglobin binds to any stray oxygen entering the nodule from air spaces and prevents it from reacting with the nitrogenase. (Like hemoglobin, leghemoglobin contains iron and turns deep red when oxygenated. In the field, scientists check for active nodules by cutting into one or two and looking for the characteristic pinkish to blood-red color of the fluids inside.)
Thousands of years ago--long before scientists understood all these details--the value of rotating legumes with other crops was recognized. The Roman poet Virgil (70-19 B.C.) recommended rotation with legumes in part 1 of The Georgics, his ode to workers of the land:
Sow in the golden grain where previously You raised a crop of beans that gaily shook Within their pods, or a tiny brood of vetch, Or the slender stems and rustling undergrowth Of bitter lupine.... Thus will the land find rest in its change of crop, And earth left unploughed show you gratitude.
Legumes continue to be of tremendous importance in agriculture, both as high-protein foods for animals and people and as "green manure" for the soil. Today, seeds of legumes sold to farmers are routinely inoculated with strains of Rhizobium selected for maximum nitrogen-fixing ability. Leguminous trees--including mesquite in the Mojave Desert, acacias in semideserts and savannas, and several tropical hardwoods--play an important role in maintaining nitrogen levels in the soils of some natural ecosystems. In some forests of Scandinavia, the Pacific Northwest, and other temperate regions, nitrogen-fixing bacteria in the genus Frankia dominate, providing nitrogen through their association with alder, bayberry, and other tree and shrub species.
Just as nitrogen fixation is energetically costly for the bacteria, it is also expensive for the plant. Symbiotic nitrogen-fining bacteria often burn up about 20 percent of the carbohydrates produced by their hosts. Preliminary results obtained by my research group at Cornell University and by others suggest some intriguing possibilities for the future. As rising levels of atmospheric carbon dioxide (associated with the burning of fossil fuels) stimulate photosynthesis, legumes will be able to generate more carbohydrates and thus support more root bacteria, perhaps gaining thereby a competitive advantage over other plants. This could be beneficial for legume crops, but it would also undoubtedly alter the mixture of species in natural plant communities and encourage the growth of leguminous weeds (such as kudzu in the southeastern United States and clovers competing with cash crops or lawn grasses).
Another area of current research is focused on the mechanisms that nature has worked out to shut down nitrogen fixation when it is not needed. We now know that some of the plant and bacterial genes controlling nodule formation and nitrogenase synthesis are turned off when large amounts of ammonia are already present in the soil.
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