How does carbon get into the soil

The age of industry

Carbon exchange with living things and soil

The Photosynthesis is the process that can convert inorganic carbon (carbon dioxide) into organic carbon (sugar) (>> more). The reverse process will be breathing called, sugar is broken down to generate energy, we exhale carbon dioxide. Most organic Carbon in living things is converted back into inorganic carbon dioxide in this way. But a part remains permanently enclosed in organic structures. This can be the case in peat bogs if plant material is not decomposed due to a lack of oxygen, but especially if organic material sinks into the sediment of the deep sea and becomes sedimentary rock over long periods of time (>> more).

The carbon dioxide in the air is connected to the organic carbon in living beings via photosynthesis and respiration. The same applies to the Carbon in the soil: It consists for the most part of dead plant material; When it is broken down, it is converted into carbon dioxide and is released back into the air. The carbon content of soils can vary greatly depending on the soil and climate; In deserts there is hardly any carbon in the soil, in deciduous forests there is a relatively high amount (in tropical climates rather less, since the degradation processes are much faster there).

If the degradation takes place in a lack of oxygen, for example in the deep sea or in swamps, organic carbon reacts with sulfate ions, producing hydrogen sulfide (H.2S). If no sulfate ions are present either, fermentation takes place during which methane (CH4) arises. In fresh water, where sulfate ions are rarer, this happens more quickly. Methane is therefore also called “swamp gas”. Methane is a highly effective greenhouse gas; in the atmosphere it is slowly oxidized to carbon dioxide under the action of sunlight; its mean lifespan in the atmosphere is 8.4 years.

Carbon exchange with the sea

in the Sea water carbon is about 50 times larger than in the air, especially than dissolved inorganic carbon: in the form of carbon dioxide, carbonic acid (H.2CO3), Carbonate (CO32-) and bicarbonate ions (HCO3-). There are also dissolved organic carbon, which, like in the ground, consists of dead living beings, and the organic carbon in marine organisms. The concentration of the dissolved inorganic carbon in sea water and the carbon dioxide in the air are in equilibrium. However, the adaptation happens relatively slowly, since the seawater is only in direct contact with the air on its surface; if the concentration changes, it can take hundreds of years to re-equilibrate (this is also the reason why seawater has absorbed less of the fossil fuel emissions than the atmosphere, although it is by far the larger carbon pool) . This exchange is of great importance in geological terms, so the fluctuations in the carbon dioxide content of the air during the cold and warm periods of the >> Ice Age are essentially caused by the oceans.

Carbon exchange with the rock

In the long run, that too will take Carbon stocks in the rock participate in the carbon cycle. When water vapor combines with carbon dioxide in the earth's atmosphere, carbonic acid is created and thus slightly carbonated rainwater. If this hits silicate rock, it reacts with it and bicarbonate ions, silicon dioxide and - depending on the rock - a calcium or magnesium ion are formed. Using the example of the silicate mineral wollastonite, it looks like this:

CaSiO3 + 2 CO2 + H2O -> approx2+ + 2 HCO3- + SiO2

Calcium and bicarbonate react to form calcium carbonate:

Approx2+ + 2 HCO3- -> CaCO3 + CO2 + H2O

In total, one carbon dioxide molecule was bound in the calcium carbonate:

CaSiO3 + CO2 -> CaCO3 + SiO2

Calcium carbonate sinks to the bottom in the ocean as sediment and over time and under the pressure of the water column and other sediments it finally turns into sedimentary rock - the original silicate rock has now become limestone. Conversely, carbon dioxide is released from limestone under the influence of heat in the earth's interior - this carbon dioxide can get back into the cycle during volcanic eruptions or in hot springs on the sea floor. The quantities of these processes are comparatively small and amount to 100 million tons per year. In geological periods, however, they help to stabilize the earth's climate (see also >> here): When the climate gets warmer, more rain falls, weathering increases and binds more of the greenhouse gas carbon dioxide - it gets colder again. If it is colder, on the other hand, the release of carbon dioxide is greater than the weathering, the concentration increases and it becomes warmer. The silicate weathering thus forms a control circuit that always directs the carbon dioxide concentration in a direction in which the weathering corresponds exactly to the release. This release of carbon dioxide is of central importance for life on earth - without it, the carbon, the central building material of >> life, would have long been completely bound in the rock. The heat inside the earth is one of the prerequisites for life on earth (>> more). However, this thermostat is very slow, so that it cannot compensate for fluctuations in concentration due to rapid changes, for example in connection with ice ages or today due to the burning of fossil fuels.

Recently, humans have massively intervened in this cycle (the red numbers in the> figure) by burning fossil fuels and burning forests to reclaim land. The carbon bound in them was released into the air, especially in the form of Greenhouse gas carbon dioxide (>> more). As a result, the carbon content in the air has increased from pre-industrial 597 billion tons to today's 820 billion tons (or better known as it is regularly read in the newspapers: the concentration of carbon dioxide from 280 ppm to 390 ppm today); the associated climate change makes the carbon cycle a focus of research on the earth's ecosystem.

Release of carbon dioxide

The carbon dioxide introduced into the atmosphere by humans is mainly due to two major sources. The first was that Slash and burn forests. More carbon is stored in a forest (in the form of wood) than in a plowed field; when forest is burned down to gain arable land, it is set free. The extraction of arable land began soon after the invention of agriculture (>> more), but increased again considerably with the technical possibilities of the industrial revolution (>> more), and has not ended to this day, especially in the tropical rainforests. In this way, about 1.6 billion tons of carbon (or just under 6 billion tons of carbon dioxide) are currently released into the atmosphere.

The other source, which is still more important today, is this Burning fossil fuels, the beginning of which also meant the beginning of the industrial revolution (>> more). This released 27.3 billion tons of carbon dioxide (or just under 7.5 billion tons of carbon) into the atmosphere in 2006. (The figure in the> figure is the average value for the years 2000 to 2005 - despite all commitments to climate protection, carbon dioxide emissions continue to rise; in 2010 they were higher than ever before in human history.)

Development of carbon dioxide emissions into the atmosphere
from 1959 to 2006 (“Other emissions” mainly include
cement production). Fig. After >> Canadell et al. 2007.

Carbon sinks

Only a part of this total remains permanently in the atmosphere, currently around 15 billion tons of carbon dioxide per year - this amount is known relatively precisely because it can be calculated from the increasing carbon dioxide concentration in the atmosphere. The “missing” amount, i.e. the difference between the released carbon dioxide and the carbon dioxide remaining in the atmosphere, is taken from the Carbon sinks recorded - ecosystems whose carbon content increases with increasing supply in the air.


The whereabouts of the carbon dioxide from 1959 to 2006. The recording
in the carbon sinks fluctuates from year to year; With
with rising emissions, an increasing share remains
in the atmosphere. Fig. After >> Canadell et al. 2007.

Land ecosystems can become a carbon sink, as climate change improves the growing conditions for plants at higher latitudes and the growing season is extended, and because plants can grow better with increasing carbon dioxide content (“carbon dioxide fertilization”) - but only if a number of other conditions are right. It shouldn't be too dry, for example, and no nutrients should be missing. On the other hand, the degradation of organic material in the soil increases with rising temperatures. It is not easy to determine which effect is how strong. Most of the carbon is found in the soils, but it is distributed very unevenly there - so the question of the informative value of the individual investigations always arises.

The simplest estimate of the carbon bound by terrestrial ecosystems is, of course, to simply determine the amount that remains in the ocean, the other major sink, and thus indirectly to infer the uptake on land. Another method consists in evaluating the carbon / oxygen ratio: When it is taken up in terrestrial ecosystems with photosynthesis, oxygen is produced - but hardly in the sea, see below. The net uptake on land is now calculated at around 900 million tons of carbon per year. Large research programs are currently under way (in Europe, for example, a program called CarboEurope, to improve knowledge about the distribution and mechanisms of this uptake.

The Uptake of carbon in the ocean is a little easier to determine because the processes are more manageable. Something lighter should not be confused with light, because the ocean is also a complex system. For example, the surface water is relatively stable separated from the deep water due to differences in density, mixing takes place primarily at the poles, where the surface water cools down to such an extent that this becomes possible (>> more). Gases from the atmosphere are first absorbed by the surface water, which is mixed by the wind down to a depth of around 100 meters. They only get into the depths when the surface water sinks there.

When carbon dioxide gets into water, carbonic acid is formed (H.2CO3). In the solution, this splits off hydrogen, so that a bicarbonate ion (HCO3-) arises. This in turn is in equilibrium with carbonate ions (CO32-). The relationship can be represented in a somewhat simplified way as follows:

CO2 + CO32- + H20 <-> 2HCO3-

If the amount of carbon dioxide increases, it reacts with carbonate ions and forms bicarbonate ions. Since carbonate ions are present in larger quantities in seawater, the seawater can absorb carbon dioxide in this way, which means that it has a high storage capacity for carbon dioxide. Up to this point, the uptake of carbon dioxide would be easy to calculate; the only unknown quantity would be the exchange between surface and deep water.

But the marine ecosystem is more complex: corals and other marine organisms need carbonate ions to form lime structures and shells. These consist of calcium carbonate (CaCO3), formed from calcium ions and carbonate ions:

Approx2+ + CO32- -> CaCO3.

When the organisms die, they sink to the bottom, thereby removing carbon from the cycle; ocean researchers call this process the "Carbonate pump". (Increasing entry of carbon dioxide leads to more and more carbonate ions being converted into bicarbonate ions, which means that the organisms lose the building material for their lime structures: Therefore, these organisms suffer from the carbon dioxide entry, which is also known as >> Acidification of the seas is known.)

Another way of binding carbon dioxide is to use small plants floating in the water, the phytoplankton: They take up carbon dioxide through photosynthesis, and after they die, some of this carbon ends up in the deep sea as a result of sinking; this is the "biological pump". The amount of carbon taken up by the ocean could be calculated in a number of ways. One is to measure the concentration of the relevant substances such as carbonate and bicarbonate ions; Another is model calculations that include the exchange of surface and deep water, which in turn is investigated by tracking rare chemicals or radioactive substances in seawater (this is how environmental pollution and nuclear weapon tests also have their useful side). The result: So far, the ocean has absorbed around 155 billion tons of carbon, more than a third of all man-made emissions. Every year the oceans take in another 2.2 billion tonnes of carbon net.

If climate change continues, this contribution could decrease: On the one hand, surface water warms up, but warm water can dissolve less carbon dioxide. On the other hand, carbonate ions are consumed in the process, so that the “carbonate pump” could lose its effectiveness. And thirdly, the sea water heats up the most at the poles, which could hinder the exchange of surface and deep water. This assumption is also supported by the fact that in recent years it has been recognized that the fluctuations in the carbon dioxide concentration in the earth's atmosphere during the ice ages were decisively caused by the carbon dioxide uptake and release of the world's oceans (see >> The Ice Ages).

Carbon dioxide uptake in terrestrial ecosystems could also be impaired by climate change, including water shortages. Most climate researchers therefore assume that the uptake by the carbon sinks will decrease in the future.

Release of methane

Human activity releases the greenhouse gas methane in greater quantities than natural sources such as swamps, oceans and others do. Methane is released during oil and gas production, when waste is fermented in landfills ("landfill gas"), when rice is grown and ruminants are raised. Methane thus contributes around 20 percent to climate change; as it oxidizes to carbon dioxide in the atmosphere over time, it does not play a role of its own in the carbon cycle.

Over a period of centuries, about three quarters of the carbon dioxide released is absorbed by seawater.Depending on how much carbon dioxide mankind releases as a whole, massive acidification of seawater would cause water to react with the sediments on the sea floor; dissolved calcium carbonate would compensate for acidification over millennia. The quarter remaining in the air is bound by the weathering of rock for thousands of years. Nature would be healed - after certainly heavy losses.

© Jürgen Paeger 2006 - 2015