Why is most of the ground brown?

Steven Allison, an ecology researcher at the University of California, Irvine, provides this answer:

Large amounts of carbon in inorganic forms absorb most visible wavelengths of light and give soils their characteristic dark brown shades. (When carbon inputs to the soil are low because of erosion or lack of plant growth, we see the red, yellow or gray hues of the underlying minerals.) The more interesting question then is: Why does soil have so much carbon?

Soils around the globe hold 1,500 to 2,300 petagrams— or as much as two quintillion grams—of carbon. That is two to three times the amount of carbon stored in the world's plants. A large fraction of this soil carbon is ancient—hundreds to thousands of years old. How is that possible when so many species of bacteria, fungi and other invertebrates decompose and consume soil carbon?

After plants die, decomposers assimilate some of their carbon and respire the remainder as carbon dioxide. When decomposers themselves die, their carbon also can be consumed and respired by other decomposers.

Several factors, however, may block the action of decomposers. Many of the microorganisms secrete enzymes to break organic compounds into small molecules, which they can then take up. But they cannot easily degrade all forms of soil carbon. Material from the cell walls of dead microbes reacts with other carbon compounds in the soil to form complex polymers. Many of these polymers are called humic compounds, and they build up in soil because their chemical structures can withstand enzymatic attacks. Along with similar molecules called polyphenols, humic compounds can bind to and inactivate the very enzymes that could potentially degrade them.

Other environmental factors diminish the efficiency of microbial enzymes. If soils are nitrogen-poor, microbes may not have the nutrients available to build enzymes. And because some enzymes require oxygen as a substrate, anoxic conditions (as in waterlogged bogs or peat lands) often cause soil carbon to accumulate.

Why do rainbows form instead of straight bands of colors? And w hy do they appear to touch the ground?

—A. WOLPIN, ATHENS, GA.

Jeff Waldstreicher, a meteorologist with NOAA's National Weather Service, explains:

Sunlight passing through raindrops creates rainbows via a process called refraction. Refraction is the bending of light as it passes from one medium to another. When sunlight hits a raindrop, it does not move as fast through the water as it does through the atmosphere, and consequently it turns a little. The light then turns again as it leaves the raindrop and returns to the air at its original speed. When light hits the rain at just the right angle, it is refracted through a raindrop and into our eyes, causing us to see a rainbow.

Because they have different wavelengths, the various colors of light refract slightly differently when they pass between mediums, which is why rainbows appear as a continuous band of colors with red on top and violet on the bottom.

A typical raindrop is spherical, and so its effect on sunlight is symmetrical about an imagined axis connecting the center of the drop and the sun. Thus, the rainbow is actually a circle centered on the point directly opposite the sun from the observer— the so-called antisolar point. We do not perceive the full circle, because the earth gets in the way. The closer the sun is to the horizon, the more of the circle we see. Right at sunset, we would observe a full semicircle of the rainbow with the top of the arch 42 degrees above the horizon. The higher the sun is in the sky, the less of the arc is visible above the horizon.

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