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Terra Obscura: The Earth's Rhizosphere
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15473 |
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Section : |
NATURAL SCIENCE
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| Issue
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12 / 1989 |
2,854 Words |
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Richard W. Zobel
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The least understood portion of our universe is not necessarily outer space--it may be the soil beneath our feet. Although we are close to the soil, it remains largely uncharted territory because it is opaque and its complex cycles are easily disrupted if the integrity of the soil is broken.
Standard scientific approaches and equipment are at best poorly suited for studying the complex and very dynamic ecology operative in the upper 6-24 inches of the earth's crust called the rhizosphere. This lack of scientific technology results in a rather fragmented understanding of rhizosphere ecology. Unseen below the surface of the soil, multitudes of plants and animals are intricately interactive with the soil with its diverse physical and chemical characteristics including temperature and moisture.
This unknown territory is emerging as the playing field upon which contend efforts to increase agricultural production (and thus the application of chemical fertilizers and pesticides) and efforts to safeguard the quality of ground and surface water. Not only is the Rhizosphere crucial to agricultural production it is also pivotal in determining the environmental consequences of applying pesticides and fertilizers. In each of the last 10 years there has been at least one major international symposium on some aspect of the rhizosphere. For example, a symposium in May 1989, "The Rhizosphere and Plant Growth," was held at the U.S. Department of Agriculture's Agricultural Research Center in Beltsville, Maryland. This symposium hosted 280 scientists from throughout the world, who attended more than 40 presentations on subjects from root growth, microbial dynamics, and substrate flow, to the use of image analysis techniques and genetic engineering of soil microorganisms. Throughout the world, most major universities and agricultural research institutes have scientists investigating aspects of the Rhizosphere. Of course, by now many specific aspects are well understood, as are many of the general relationships. This enables scientists to achieve better control of diseases and pathogenic organisms, as well as to make decisions on cultivation and application of fertilizer, herbicide, and insecticide. Yet overall, the complex interactions occurring in this ecological niche remain poorly understood.
What Is The Rhizosphere?
Rhizosphere has many definitions, and each one is characteristic to a specific research project. In the broadest sense, the Rhizosphere is that region of the soil containing plant roots and organisms ranging in size from viruses to single-celled organisms to mice and even gophers. The Rhizosphere is the forgotten part of the biosphere, being the transition zone in which the visible biosphere meets the earth's crust.
Depending on such factors as moisture, temperature, previous plant populations, and depth to solid rock or a water table, the Rhizosphere can vary from several inches to several feet in thickness. Yet the Rhizosphere is any especially difficult territory for scientific exploration, because its major component is the soil, a generally dense, opaque substance through which light and other electromagnetic waves pass only with extreme difficulty, it at all. Also, its component parts range in size from microns to square yards, making it difficult to study a single part without destroying the whole. These factors have inhibited scientific study of the rhizosphere, and most people assume that little is
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