As part of the carbon cycle, leaves decompose and the carbon in their bodies is broken down and recycled. Some of it is released into the air as carbon dioxide, or CO2. The rest moves into the soil.

Each year, spring comes, plants bloom and the trees leaf out in their full green glory. Come fall, while diving into piles of fallen leaves, you may think the life cycle of the leaf has come to an end.

But that’s not so. Once a leaf hits the dirt, a new cycle begins. All those brightly colored leaves are like candy for fungi and bacteria on the ground. These decomposers, organisms that feed on dead matter, go to work breaking down leaves to create energy-filled food for themselves. In the process, decomposers also make nutrients available for other organisms.

This recycling scheme is not just a plot to produce a mob of mushrooms and other eensy entities. It’s part of a complex chemical cycle that helps regulate the Earth’s climate. And it’s all based on carbon, a kind of element, or tiny substance.

Carbon is the building block for all life on Earth. Every single cell in every living thing — including plants, animals and humans — contains at least some of the stuff.

Carbon isn’t found only in living matter. It’s also found inside the Earth’s mantle, the layer between the crust and the core, and in seawater, air, rocks and soil. The planet’s carbon is constantly flowing from one of these to another, creating what is known as the carbon cycle.

Take those leaves, for example. As they decompose, or rot, the carbon in their bodies is broken down and recycled. Some of it is released into the air as carbon dioxide, or CO2. The rest moves into the soil.

Soil is a great place for carbon. There, it may remain locked up for hundreds, thousands or even millions of years, adding nutrients needed for growing food. Keeping carbon locked up in the soil also provides a way to keep it out of the atmosphere.

Dig in

Carbon has a very complicated cycle within the soil and in the atmosphere. The two cycles are intricately linked, says Patrick Drohan, a pedologist (scientist who studies soil) at Pennsylvania State University in University Park.

Though some of the carbon in soil comes from sedimentary rocks, such as limestone, most of it comes from organic matter, meaning waste from living organisms. Sounds a bit yucky, but it’s really cool. He explains the cycle like this:

A squirrel poops (or a plant or animal dies) and the waste then decomposes. Nutrients in the organic matter, including carbon, are released into the soil with the help of decomposers such as fungi and bacteria. Over the years, the nutrients are broken down further. Eventually, the nutrients get reabsorbed by a plant taking up water, or a human eating food grown in the soil or perhaps by a tiny organism called a microbe within the soil. When that microbe breathes, it releases CO2 into the atmosphere. Plants absorb the CO2 released from the microbe. From here, the cycle begins again.

Leaves on the ground are like candy for fungi and bacteria. These decomposers, organisms that feed on dead matter, go to work breaking down leaves to create energy-filled food for themselves. In the process, decomposers also make nutrients available for o

Karl Lipschitz/stock.xchng

Concern over the rapid buildup of carbon dioxide in the atmosphere has prompted scientists to look at ways to sequester, or contain, carbon in the soil and plants. The key to doing this is plant production.

Scientists say promoting and protecting the growth of forests and other plants may boost plants’ capacity to take up CO2 in the atmosphere. Such practices may also increase soils’ capacity to store carbon for long periods of time.

The power of plants

Most of the carbon on Earth is stored in plants and soil.

Where does all this carbon come from? Plants get all of their carbon from carbon dioxide, or CO2, in the atmosphere. The leaves on trees and crops soak up CO2 during photosynthesis, a chemical process that converts sunlight into food. Then plants spit some of the CO2 back out during another process called respiration, the way plants “breathe.”

Plants, especially trees, are so efficient at pulling carbon dioxide from the air that they take in more carbon than they release. That’s why they’re called “carbon sinks.”

Trees grouped together in forests are even more efficient. Scientists estimate that the Earth’s forests currently store more than 75 percent of the planet’s aboveground carbon. And the forests store almost that much of the planet’s soil carbon.

Scientists are working to develop forest management strategies to help absorb some of the extra CO2 in the atmosphere. But this task isn’t as straightforward as it may seem.

Not all forests actually store carbon, says Peter Curtis, a forest ecologist at Ohio State University in Columbus who studies the role of forests in the carbon cycle. “Some forests experience a net loss.”

That doesn’t mean that the trees have stopped photosynthesizing. It simply means that the respiration part, the loss, is greater than the gain, he explains.

Accounting for carbon

Curtis works to measure how much carbon can be held in forests in the Midwest and Great Lakes region. Working from the University of Michigan Biological Station in northern Michigan, he has two ways of doing that.

First, he uses a high-tech approach: Information is collected on and around two meteorological, or weather-measuring, towers, which look a lot like cell phone towers. Standing 150-feet-tall — about as high as a 15-story building — the towers loom over the forest’s canopy.

Instruments on the towers measure how much CO2 is being taken up by the leaves on the trees. The instruments also measure temperature and moisture levels in the air, recording information up to 10 times per second.

The scientists also use some “low-tech” methods to collect data. In other words, researchers spend lots of time on the ground measuring the trees and collecting leaves to see how much debris has decomposed.

Using this information, Curtis tracks how much carbon the forests take in through photosynthesis, and how much they lose through respiration.

Carbon is the building block for all life on Earth. Every single cell in every living thing — including flowers, frogs and humans — contains at least some of the element.

Keith Weller/USDA-ARS

“It’s like a bank account,” he says. “If you get $10 in allowance, but have $8 in expenses, then $2 is what goes into your account.”

The trees may take up a ton of CO2 per acre, but respire 1,500 pounds, leaving a “profit” of 500 pounds of carbon intake.

Fortunately, most forests take in more carbon than they loose. Generally speaking, the planet’s forests take in about 25 percent of the CO2 created by human activities, Curtis says.

Areas heavily populated with forests absorb even higher amounts of human-generated CO2. In some parts of Michigan or Maine, the oaks and pines found in hardwood forests take up about 60 percent of the carbon emitted by people that live in that area.

“A forest in one of these areas can soak up the yearly emissions of about 225,000 cars,” Curtis says. “We call that an ecological forest.”

But changes in rainfall and temperature can shift a forest’s ability to hold carbon from year to year. Unseasonably warm temperatures in a cool, wet forest, for example, can speed the rate of decomposition of soil matter. When that happens, carbon that has been stored in the soil for hundreds, even thousands of years, may be released back into the atmosphere.

Such changes have been documented in some Canadian forests, Curtis says. “This is one of the big worries with climate change. When temperatures increase, decomposition ramps up and the forest gets drier, and all that soil carbon starts to be lost.”

Small changes

Scientists don’t yet know all the effects climate change will have on soil’s ability to store carbon, Drohan says.

They do know, however, that even a small change in soil carbon storage can have a significant impact on the global carbon balance. To that end, researchers are looking at ways farmers might better manage their crops and soil.

Practices designed to keep carbon in the soil will benefit farmers, as well as the planet. Carbon adds organic matter, which helps soil retain nutrients and water. Soil carbon also improves the structure of soil, resulting in better drainage and aeration, or flow of gases, for roots. That means healthier plants and better yields for farmers.

You don’t have to be a farmer to benefit, or to help. Curtis spends some of his time working with government officials and landowners to help them manage forest areas for the benefit of the planet and its soil.

At Michigan Technological University, faculty and students are leading a community effort to return carbon to the soil. The group throws logs and other debris into a large container. These scraps are then burned slowly at a low temperature to create bioch

Michigan Technological University

Even small-scale, community efforts can help. At Michigan Technological University, faculty and students are leading a community effort to return carbon to the soil. Instead of just letting agricultural and plant wastes degrade on their own, the group throws logs and other debris into a large container. These scraps are then burned slowly at a low temperature.

This smoldering process produces a substance called biochar that resembles the char left by a campfire. More importantly, the slow burn prevents much of the carbon from getting released back into the air, says Michael Moore, who’s leading the effort. The char can then be tilled right into the soil, where the carbon stays locked for years.

Amazonian natives have used this technique for centuries to fertilize their soil, says Moore, who teaches writing and poetry. He learned about it while traveling in Honduras.

Biochar isn’t ready for large-scale agriculture yet, but Moore says such community efforts provide a way for ordinary citizens to help the planet. And that has benefits for all.

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Word Find: The Dirt on Carbon

New England’s brilliant fall colors attract many visitors.

National Park Service

“Being in the Northeast during autumn is just about as good as it gets in this country,” says David Lee. He’s a botanist at Florida International University in Miami.

Lee studies leaf color, so he’s biased. But plenty of other people share his admiration. Areas of the United States with especially colorful fall displays attract thousands of leaf peepers.

Even as they “ooh” and “aah,” few people know what makes many plants blush in the autumn. Research has shown that leaves change color when their food-making processes shut off. The chemical chlorophyll, which gives leaves their green color, breaks down. This allows other leaf pigments—yellow and orange—to become visible.

No one knows exactly how global warming will alter forests and affect fall colors.

J. Miller

But “there’s still a lot we don’t know about this,” Lee says.

It isn’t clear, for example, why different species of plants turn different colors. Or why some trees become redder than others, even when they’re standing right next to each other. And no one knows exactly how global warming will alter forests and affect leaf-peeping season.

Food factory

In summer, when a plant is green, its leaves contain the pigment chlorophyll, which absorbs all colors of sunlight except green. We see the reflected green light.

The plant uses the energy it absorbs from the sun to turn carbon dioxide and water into sugars (food) and oxygen (waste). The process is called photosynthesis.

When chlorophyll breaks down, yellow pigments in leaves become visible.

I. Peterson

As days get shorter and colder in the autumn, chlorophyll molecules break down. Leaves quickly lose their green color. Some leaves begin to look yellow or orange because they still contain pigments called carotenoids. One such pigment, carotene, gives carrots their bright-orange color.

But red is special. This brilliant color appears only because the leaves of some plants, including maples, actually produce new pigments, called anthocyanins.

That’s a strange thing for a plant to do without a reason, says Bill Hoch of the University of Wisconsin in Madison. Why? Because it takes a lot of energy to make anthocyanins.

Why red?

To figure out the purpose of the red pigment, Hoch and his coworkers bred mutant plants that can’t make anthocyanins and compared them with plants that do make anthocyanins. They found that plants that can make red pigments continue to absorb nutrients from their leaves long after the mutant plants have stopped.

Red leaves get their color from a pigment called anthocyanin.

I. Peterson

This study and others suggest that anthocyanins work like a sunscreen. When chlorophyll breaks down, a plant’s leaves become vulnerable to the sun’s harsh rays. By turning red, plants protect themselves from sun damage. They can continue to take nutrients out of their dying leaves. These reserves help the plants stay healthy through the winter.

The more anthocyanins a plant produces, the redder its leaves become. This explains why colors vary from year to year, and even from tree to tree. Stressful conditions, such as drought and disease, often make a season redder.

Now, Hoch is breeding plants for a new set of experiments. He wants to find out whether turning red helps plants survive cold weather.

“There’s a clear correlation between environments that get colder in the fall and the amount of red produced,” he says. “Red maples turn bright red in Wisconsin. In Florida, they don’t turn nearly as bright.”

More protection

Elsewhere, scientists are looking at anthocyanins in other ways. A recent study in Greece, for instance, found that as leaves grow redder, insects eat them less. On the basis of this observation, some scientists argue that red pigments defend a plant against bugs.

Leaves may turn red in the autumn to protect themselves from the sun’s ultraviolet rays.

J. Miller

Hoch rejects that theory, but Lee thinks that it might make sense. He points out that red leaves contain less nitrogen than green ones do. “It may actually be that insects avoid red leaves because they’re less nutritious,” Lee says.

However, “it’s pretty confusing at this point,” Lee admits. “People debate back and forth.”

To settle the debate, scientists will need to look at more species under more conditions, Lee says. So, he’s now researching leafy plants rather than trees. He’s especially interested in tropical plants, whose leaves turn red when they’re young rather than old.

You can do your own leafy experiments. Observe the trees in your neighborhood and keep track of weather conditions. When autumn begins, write down when the leaves change, which species change first, and how rich the colors are. You can even see anthocyanins under a simple microscope. After several years, you might start to notice some patterns.

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Word Find: Leaf Color

Now, a geologist from the Field Museum in Chicago says that she has found a new way to figure out how the shape of Earth’s surface has changed over time. Her strategy? Leaf peeping.

Pores that take in carbon dioxide appear in this microscopic view of a present-day California black oak. The sample was stained orange so that details would be more visible.

Jennifer C. McElwain, Field Museum

A tree’s leaves have tiny holes called stomata. These pores allow the leaves to take in a gas called carbon dioxide, which the tree needs in order to survive.

With this fact in mind, geologist Jennifer McElwain collected leaves from living California black oak. These trees grow in a wide range of altitudes, from sea level all the way up to 2,500 meters (8,200 feet).

McElwain used a microscope to count how many stomata were inside a given area of each leaf. She found that the leaves had more stomata at higher altitudes. Then, she came up with an equation that links stomata numbers and elevation.

The black oak has been around for at least 24 million years. So, scientists can now count stomata on fossilized leaves to figure out how high the trees were when they lived, McElwain says. By comparing this altitude with the altitude at which the fossils were collected, the researchers can measure any changes in elevation that had occurred.

The new method should be more accurate than previous methods, McElwain says. Next, she wants to come up with equations for other tree species.

Someday, she says, her research may help scientists answer a major question in geology: When did the Himalayas in Asia rise?—E. Sohn

Going Deeper:

Shiga, David. 2004. Ancient heights: Leaf fossils track elevation changes. Science News 166(Dec. 18&25):390. Available at http://www.sciencenews.org/articles/20041218/fob7.asp .

You can learn more about leaf pores (stomata) at www.microscopy-uk.org.uk/schools/images/stomata.html (Microscopy-UK) and www.accessexcellence.org/AE/AEC/AEF/1994/case_leaf.html (National Health Museum).

Information about the geology of the Himalayas can be found at jan.ucc.nau.edu/~wittke/Tibet/Himalaya.html (Northern Arizona University).