Environmental Benefits of Carbon Sequestration:

Facts About the Carbon Cycle and Greenhouse Gases

If more greenhouse gases are stored in soil, how will this affect the natural carbon cycle and global climate change?

Greenhouse gases consist primarily of carbon dioxide (CO₂), methane, nitrous oxides, and water vapor, says Richard Nelson, K-State Extension energy specialist. Of these, water vapor is the most prevalent, but it is the other gases that are increasing and causing problems.

“None of these gases is toxic in any way, but they can disrupt the natural climate balance if their atmospheric levels are too high. These atmospheric gases are called greenhouse gases because they trap heat radiating off the earth and hold it in the atmosphere near the surface,” Nelson explains.

What are the sources of greenhouse gases? The primary sources of CO₂ are fossil fuel burning, the natural decomposition of organic materials by microorganisms, and animal respiration. The primary sources of methane are fossil fuel burning and animal waste gases. The primary sources of nitrous oxide are nitrogen fertilizer use and cars with catalytic converters.

Many people have heard of chlorofluorocarbons (CFCs), too. This gas, used as a refrigerant and propellant for many years, causes damage to the ozone layer in the upper atmosphere. This is a separate issue from global warming.

Of all the greenhouse gases, it’s the level of increase in CO₂ that is perhaps of most concern. Buildup of atmospheric CO₂ indicates a disruption in the natural carbon cycle.

The natural carbon cycle has been disrupted during the last 200 years by the dramatic increase in fossil fuel emissions, says Chuck Rice, K-State soil microbiologist. The effect of this disruption may be global warming, according to many scientists. Scientists at K-State and other midwestern universities are attempting to reverse some of this disruption by storing more greenhouse gases, such as carbon dioxide (CO₂), in soil.

 “Atmospheric CO₂ levels have risen nearly 25 percent over just the last 200 years. This is a tremendous increase in a very short period of time, and a disturbing trend for those concerned about global warming. We need to act now in order to restore the carbon cycle to a more normal balance,” Rice says.

It is critical to life on Earth that the carbon cycle remains in proper balance, Rice explains. “The carbon cycle acts as a governor on the Earth's environment, preventing CO₂ levels in the atmosphere from getting too high or too low. If CO₂ levels become too high, global warming could occur. If CO₂ levels become too low, global cooling could occur. Within an acceptable range of CO₂ levels, environmental swings remain moderate enough to sustain life as we know it,” he says.

On our neighboring planets, Venus and Mars, the carbon cycle does not function as it does on Earth. In the case of Venus, too much of the carbon remains in the atmosphere as CO₂ and temperatures at the planet's surface are unbelievably hot. In the case of Mars, too much of the carbon remains fixed in rocks. There is very little CO₂ in the atmosphere on Mars and as a result, the atmosphere cannot hold what little heat it receives from the sun. 

The basic carbon cycle of life is: (1) the conversion of atmospheric carbon dioxide to carbohydrates by photosynthesis in plants; (2) the consumption and oxidation of these carbohydrates by animals and microorganisms to produce carbon dioxide and other products; and (3) the return of carbon dioxide to the atmosphere. On a global level, the total carbon cycle is more complex, and involves carbon stored in fossil fuels, soils, oceans, and rocks.

We can organize all the carbon on earth into five main pools, listed in order of the size of the pool:

1. Lithosphere (Earth's crust). This consists of fossil fuels and sedimentary rock deposits, such as limestone, dolomite, and chalk. This is far and away the largest carbon pool on earth. The amount of carbon in the lithosphere: 66 to 100 million gigatons (a gigaton is one billion metric tons). Of this amount, only 4,000 gigatons consists of fossil fuels.
2. Oceans. Ocean waters contain dissolved carbon dioxide, and calcium carbonate shells in marine organisms. Amount of carbon: 38,000 to 40,000 gigatons.
3. Soil organic matter. Amount of carbon: 1,500 to 1,600 gigatons.
4. Atmosphere. This consists primarily of carbon dioxide, carbon monoxide, and methane. The amount of carbon in the atmosphere has increased from 578 gigatons in 1700 to about 766 gigatons in 1999, and continues to increase at the rate of about 6.1 gigatons per year.
5. Biosphere. This consists of all living and dead organisms not yet converted into soil organic matter. Amount of carbon: 540 to 610 gigatons.

Carbon moves back and forth among these various pools, Rice explains. Nearly all of the carbon on earth is locked up in the lithosphere as sedimentary rock deposits and fossil fuels. And about 99.999% of this carbon is fixed in place and essentially off the table as far as the carbon cycle is concerned. Only the amount stored as fossil fuels enters the carbon cycle, and only then through human activities.

A sizable percentage of the "free" carbon on Earth exists in the atmosphere. As the carbon cycle undergoes shifts and fluxes through the eons, the amount of carbon in the atmosphere tends to increase or decrease to buffer the changes. Currently, the atmospheric carbon pool is expanding by about 6.1 gigatons per year, and the fossil fuel carbon pool is shrinking by about 4 to 5 gigatons per year. This is one aspect of the carbon cycle that can be readily manipulated by human activity.

The ocean absorbs 2.5 gigatons of carbon more from the atmosphere than it gives off to the atmosphere. But that extra amount of carbon is utilized by marine biota and eventually gets incorporated into deep sea deposits and sediments. So the net level of carbon in the ocean remains roughly the same every year.

The soil organic matter pool is currently losing about 1 to 2 gigatons of carbon per year to the atmospheric pool, Rice says. About 60 gigatons of carbon per year enters the soil organic carbon sink as decaying biomass remains in the soil. About 61 to 62 gigatons of carbon are lost from this pool as soil organic matter is oxidized by the atmosphere. This is the other main cycle that can be manipulated by human activity.

“Changes in land use patterns and agricultural practices can affect the amount of carbon released into the atmosphere from soil organic matter,” Rice says.

The biosphere represents a significant carbon pool on Earth. About 110 gigatons per year of carbon is absorbed by the atmosphere into plant life through the process of photosynthesis. Of that amount, about 60 gigatons of carbon is released into the atmosphere through respiration, decay, and gaseous waste elimination from living animal biomass, both on land and in the ocean. The other 50 gigatons is incorporated into soil organic carbon, part of which can be readily oxidized and part of which is relatively stable for many years.

Before the industrial revolution, the main source of fluctuation in atmospheric carbon was from changes in biomass and soil organic carbon. Now, fossil fuel burning is the greatest factor in atmospheric carbon fluctuations.

The bottom line of all this is that the amount of carbon in the atmosphere is increasing by about 6.1 gigatons per year, mostly due to fossil fuel burning and land use changes that destroy soil organic carbon. This increase needs to stop, or at least slow down, since carbon dioxide in the atmosphere traps heat and becomes a greenhouse gas that can lead to global warming.

The atmospheric carbon balance sheet looks like this:

Factor	Carbon flux into atmosphere (gigatons C/year)	Movement of C out of atmosphere (gigatons C/year)
Fossil fuel burning	4-5
Soil organic matter oxidation/erosion	61-62
Respiration from organisms in biosphere	50
Deforestation	2
Incorporation into biosphere through photosynthesis		(110)
Diffusion into oceans		(2.5)
Net	117-119	(112.5)
Overall Annual Net Increase in Atmospheric Carbon	+4.5-6.5

So how can humans manipulate the carbon cycle so that the atmospheric carbon pool stops expanding?

“Let's look at the two ends of the equation: where atmospheric carbon goes, and where it comes from. This will give us an idea of what changes can be made to reduce carbon buildup in the atmosphere,” Rice says.

Where atmospheric carbon goes.

1. Diffuses into the ocean. This part of the carbon cycle is difficult to manipulate.
2. Into plant life. This can be increased by increasing plant growth through reforestation, changes in agricultural cropping systems, and reclamation of marginal land.
3. Into soil organic carbon. As plant life decays, part of its carbon is converted by microorganisms into soil organic matter. In the initial phases of this process, the organic matter is in a "short-term" pool and can be easily oxidized. Once this happens, the carbon is released back into the atmosphere. By changing agricultural practices, it is possible to increase the amount of soil organic carbon in the intermediate and long-term pools.

Where atmospheric carbon comes from.

1. Fossil fuel emissions. This is the fastest increasing source of carbon buildup in the atmosphere.
2. Soil organic carbon destruction. Through excessive tillage and soil erosion, soil organic carbon can be oxidized and lost to the atmosphere. The total amount of carbon stored as soil organic carbon is roughly equal to the sum of the amount in the atmosphere plus the amount existing in plant and animal life combined. So any changes in soil organic matter destruction or creation can have a significant impact on atmospheric carbon levels.
3. Deforestation. As forests are burned for land clearing or other reasons, a significant amount of carbon is released into the atmosphere.

For more information, contact:

Dr. Charles W. Rice

Department of Agronomy

Kansas State University

Manhattan, KS 66506

(785) 532-7217

cwrice@ksu.edu

Dr. Richard Nelson

Extension Energy

Kansas State University

Manhattan, KS 66505

(785) 532-4999

rnelson@ksu.edu