The Biosphere Puts It Together Study Guide (page 3)

Updated on Sep 26, 2011


The air contains two major forms of carbon: carbon dioxide (CO2) and methane (CH4). CO2 is 0.037% carbon and CH4 is 0.00014%; thus, CO2 is the dominant form. What is the total tonnage of carbon in the biosphere's atmosphere? The calculation requires knowing the total mass of the atmosphere and the molecular weights of the various gases, but let's go directly to the answer, rounded off: about 700 billion tons of carbon. It is intriguing that the amount of carbon as carbon dioxide gas in the atmosphere is about the same as the amount of carbon in the form of organic molecules in all of life globally.


Most of the carbon in the soil occurs in the top two layers, the O and A layers, which are the layers of organic litter and topsoil, respectively. Here we count only the carbon in the form of organic matter, because that is the form circulated by life's activities in the soil. Carbon also exists within certain minerals in the soil (such as carbonates), and that varies tremendously from place to place. The carbon in the organic matter of the world's soils is about three times the amount of carbon in the atmosphere, for a total of about 2,100 billion tons of carbon.

The oceans contain by far the largest reservoir of carbon. In addition to invisible amounts of organic carbon (dissolved proteins and carbohydrates from organisms, for example), seawater has carbon in other chemically active forms. One form is simply dissolved carbon dioxide gas, but this reacts with the water molecules to form the two other main types of carbon containing chemicals in seawater. The first is the bicarbonate ion (single negative charge), and the second is the carbonate ion (double negative charge). Of all the types of carbon in seawater, the one with the largest amount is the bicarbonate ion. All told, carbon in the ocean totals a whopping 35,000 billion tons.

Now we come to the fluxes of carbon between the reservoirs. Recall that water changes state as it moves between some of its reservoirs in the global water cycle, changing, for example, from liquid to gas as it moves from ocean to atmosphere during evaporation. Carbon is much more complicated because carbon changes the atoms it is bonded with into molecules during some (but not all) of its movements between reservoirs.


The major fluxes between life and air are driven by land organisms. Trees and grasses move about 60 billion tons of carbon per year from its gas state as carbon dioxide in the atmosphere into the various organic molecules inside plants. The process is called photosynthesis. In contrast, animals such as flying insects and deer, which consume and digest plants, thereby releasing the carbon from its organic form back into carbon dioxide, create a flux of only about 5 billion tons per year. This release of carbon dioxide by animals takes place during their metabolic process of respiration.


Photosynthesis and respiration also occur in the ocean between the various forms of carbon in seawater and the organic carbon in marine organisms. Estimates peg both processes within the ocean at about 40 billion tons of carbon per year.


The flux between life and the ocean seems in balance, with photosynthesis and respiration at about 40 billion tons a year. But the exchange between life and air on land seems out of balance. Land plants remove 60 billion tons of carbon per year, but land organisms (we mentioned deer and flying insects) return only 5 billion tons per year. What happens to the other 55 billion tons per year taken out of the air by photosynthesis on land? The answer is that much of the growth of plants falls to the soil. How much? About 55 billion tons per year. This yearly flux of detritus to the soil provides the food for the soil organisms (not included in the organisms that live above ground, such as flying insects and deer). Organisms such as worms and bacteria in the soil consume the organic matter in the soil and create carbon dioxide waste during their own processes of respiration, which occur underground. Roots of plants also contribute to this respiration flux of CO2 to the soil.


The 55 billion tons of carbon dioxide gas added into the air pockets of the soil each year have to go some-where. It travels up into the atmosphere in the process of gas exchange between the soil and atmosphere. The net result of this gas exchange is that carbon dioxide moves from the soil into the atmosphere in a flux of 55 billion tons of carbon per year.


As noted earlier, the main flux between the atmosphere and ocean occurs as gas exchange. In the global biogeochemical cycle of carbon, carbon dioxide moves back and forth between the ocean and atmosphere. The flux is about 100 billion tons per year. In the natural system of the biosphere, this back-and-forth flux is in balance. Today, because humans are adding car bon dioxide to the atmosphere during the combustion of coal, oil, and natural gas, a slight net flux (about 2% excess) is going into the ocean, compared to the carbon dioxide that is leaving the ocean into the atmosphere.

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