Steven Allison, an ecology researcher at the University of California, Irvine, provides this answer.
Many soils are brown in color because they contain large amounts of carbon. In particular, carbon-containing polymers called humic compounds absorb most visible wavelengths of light and give soils a dark brown appearance. Often the majority of soil carbon is present as humic compounds, which means they have a large impact on soil chemistry and fertility.
What is most surprising about humic compounds, and indeed all soil carbon, is that there is so much of it. Many species of bacteria, fungi and other invertebrates decompose and consume soil carbon as a food source, yet soils hold somewhere between 1,500 and 2,300 petagrams--or as much as two quintillion grams--of carbon globally; this is two to three times the amount of carbon present in all the plants in the world. A large fraction of this soil carbon is ancient--hundreds to thousands of years old--meaning that it has escaped conversion into carbon dioxide by soil decomposers. These escape mechanisms are ultimately what cause the ground to be brown.
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Ecologists have long wondered how plants avoid being eaten by herbivores, that is: Why is the world green? Yet few have asked the analogous question about carbon in the soil. It turns out that chemistry explains why herbivores don¿t eat some plants and why so much soil carbon escapes decomposition. The chemical challenges are especially acute for decomposers, because so many of them are microorganisms that cannot take up their food directly. Instead, they secrete enzymes to break down organic compounds into small molecules that they can take up. If these enzymes are intercepted or destroyed in the soil environment, then decomposition slows down.
Even when microbial enzymes persist in the soil, they are not capable of degrading all forms of soil carbon. Soils represent the final destination for carbon fixed by plants during photosynthesis. After plants die, decomposers consume the dead plant carbon and assimilate some of it while respiring the remainder as carbon dioxide. When the decomposers die, their assimilated carbon can be consumed and respired by other decomposers. Over time, this recycling process returns most of the carbon to the atmosphere as carbon dioxide, but a small fraction is transformed into chemically resistant forms that accumulate in the soil. These compounds no longer resemble plant material at all but rather are the chemical leftovers of decomposition. Many are humic compounds and their complicated chemical structures prevent enzymes from efficiently attacking them. Along with chemically similar compounds called polyphenols, humic compounds act as a true dead end for soil carbon because they can also bind to and inactivate the very enzymes that could potentially degrade them.
Other environmental factors also diminish the efficiency of microbial enzymes. If soils are nitrogen poor, then microbes may not have the nutrients available to build enzymes. Some enzymes require oxygen as a substrate, thus anoxic conditions often cause soil carbon to accumulate; this occurs in many bogs and peatlands because of their waterlogged soils. Also, many soil minerals adsorb enzymes and soil carbon, including humic compounds. This process blocks the enzymes from achieving the correct orientation to attack their carbon substrates.
Not all ground is brown, of course: soil minerals, when not covered in carbon compounds, often give soils a red, yellow or gray hue. In some ecosystems, we see the colors of the underlying minerals instead of brown ground, because carbon inputs to the soil are low due to erosion or a lack of plant growth, as in the iron-rich red soils of certain deserts. Yet, ultimately, the majority of ground is brown because the majority of soils remain carbon-rich.
