Carbon sequestration by soils

Global warming is mainly caused by the increase of CO2 (carbon dioxide) concentration in the atmosphere, although other greenhouse gases also contribute to climate changes, like methane (CH4), which is even a more potent greenhouse gas than CO2. Both these gases originate from the burning of carbonaceous compounds, which may be fossil or modern in origin. This burning may be biological, in which case it is the product of respiration, or man made, as in the burning of oil or coal.
Most biological burning of carbon compounds comes from the decomposition of organic matter by microorganisms, such as bacteria and fungi, which act mostly upon oganic matter originated from plants. Some of this organic matter is highly resistant to microbial decomposition and ends up, after yet unclear biochemical changes, as soil organic matter, also known as humus or humic substances.
To prevent global climate changes, a number of measures have been suggested to slow or even stop CO2 increase in the atmosphere, from planting (and conserving) forests to fertilizing the ocean with iron to cause algal blooms. These measures would increase the carbon sequestered as plant biomass, since photosynthesis is basically the uptake of CO2 from the air to build plant tissues utilizing energy from the sun. What most people don’t know is that there is much more carbon stored as soil organic matter than as standing vegetation biomass.
There is possibly more carbon stored in some soils of the Amazon Rain Forest than in the forest itself! Current estimates for carbon stored in terrestrial vegetaion are 550 billion metric tons, while soils store at least 1200 billion metric tons, more than twice as much. What’s more, soil carbon storage is probably underestimated, because the carbon in deep layers of some tropical soils is generally not considered in soil carbon stocks estimates.
Carbon dioxide emitters from developed countries can balance their emissions by buying carbon credits from developing countries. Generally, these carbon credits are in the form of planting forests or paying to conserve them. Obviously, this is a good arrangement for both. But those who sell carbon credits may be losing money, as the planting of forests generally increase the carbon content in soils, which is not being considered. Soil organic matter is more stable than the organic matter in vegetation, so it’s more efficiently sequestered.
What some people do not understand about some tropical soils called Oxisols is that they are deep. These soils are the result of centuries of intense weathering, especially chemical weathering, under high temperatures and rainfall. Organisms also play an important role in the formation of these soils, notably plants exuding organic acids and other substances. The activity of mesofauna in these soils is impressive, and termites, earthworms and ants are very active in mixing soil material throughout the soil profile, making the Oxisols rather homogeneous vertically.
A large area of tropical rain forests, including Amazonia, is on Oxisols. Until recently, soil carbon stocks estimation only considered soil depth down to around one meter. It happens, though, that Oxisols may be much deeper than that and they can and do store organic carbon in deep subsurface horizons, which are more or less horizontal layers in the soil profile. In fact, subsurface carbon stocks in Oxisols may be twice the surficial stocks, as I found in my doctorate research.
Recent research has demonstrated that deep soil carbon is very stable, which is good when one considers carbon sequestration. Besides, this carbon is spacially separated from the agents that could decompose or mineralize it. These soils also have good natural physical characteristics, which can be maintained even after land use changes if they are properly managed, so greatly decreasing erosion risks. All this, in our view, make tropical Oxisols promising sinks for carbon sequestration and medium to long term storage in helping decrease atmospheric CO2 excess concentrations.

Discussão - 12 comentários

  1. Paula disse:

    OK, OK… but is it possible force CO2 in to the soil, like inject carbon dioxide in there? Or the idea is that the carbon dioxide is consumed by the plant, that release the leaves and than, the carbon is absorbed by the soil?
    Another question: How to calculate the amount of carbon storage in a soil?

  2. Paula,
    The idea is to improve soil conditions so that it can more efficiently store the organic carbon fixed by plants. Soil organic matter is really a complex mixture of several different compounds, some produced by plants, some by microorganisms and some by biochemical reactions in soils. They will have different decomposition resistance, but the most refractory are the very complex humic substances. To calculate carbon soil stocks you need the soil organic carbon concentration (which is something rather simple to determine), the thickness of the soil layer you sampled and the soil bulk density. As to injecting CO2 into the soil, Idon’t think it”s an effective way to sequester carbon: besides being too porous a medium, it is too clos to atmosphere and, really, soil CO2 is in equilibrium with atmospheric CO2.

  3. manuel disse:

    Dear Italo
    I did not know the oxisols were so important as to CO2
    sequestration. Thank you for this information.
    But the fact is the oceans have a more important role in this regard. For this reason,as you noted,there are steps to increase this important capacity.
    As you know also,there are other means to capture CO2 in excess,such as injecting in deep ocean or iced sites. Artificial trees,cement production,calcareous or marble formation through peridodite,sites already without crude or gas,are being also considered,so grate is the preoccupation as to CO2 in excess,this CO2 so fundamental to life. But it is the life,as we say. And at same time the excellent forest has been reduced!
    Good health,and many thanks for this opportunity to brush my very poor english.

  4. Luiz Bento disse:

    The ocean is by far the most important carbon storage. And it is constant absorbing carbon from the atmosphere. The problem is that free co2 is acid. More co2 absorption, more acid the ocean becomes. The carbon stored as calcium bicarbonate can be lost due to this acidification, transformed in carbonic acid. That is really a big problem and the tipping point is not so far away.

  5. Gentlemen,
    I did not state the soil is the larger carbon sink nor the most important one in the global carbon cycle, I said that in the terrestrial compartment, soil is a larger sink than vegetation, that is a widely recognized fact.

  6. manuel disse:

    Dear Ítalo
    You are right. The question you put it is a particular one. There are several CO2 sinks,happily,as all we know.
    In relation to this, the following site seems to me interesting-
    As to oxisols, they deserve indeed our gratitude for the role they have as a great CO2 sink.

  7. manuel disse:

    Dear Luiz
    A simple observation. Ca bicarbonate is completely soluble,as you know. The entities are,as you know, also,the negative ion bicarbonate and the positive ion calcium.When the acidity increases with more CO2 in the water,as you said,more bicarbonate ion is formed,that it is acid yet. The calcium is another history. It comes,more and more,as other cation,because the water of the rivers bring them since the beginning. So,when the solubility product of the various salts is reached there is precipitation,in calcium carbonate,calcium sulphate,or other,in calcium case.It is clear that the ocean water also have
    its part, dissolving components of the soil in contact.A very complicate thing,for a curious like me.
    I suppose this it is correct. If not,I present my many excuses.
    Um abraço

  8. Luiz Bento disse:

    You are correct manuel, of course. I just jumped from the one point to another. I thought that was no need to make another post about carbon balance in the ocean. The whole balance depends on pH. And the important thing is that pH changes could change the carbon forms in the ocean, releasing the major part of the carbon that was unavailable for remineralization.

  9. Paula disse:

    I´m not sure, but I think there is a little confusion on the ocean point.
    The ocean is a very good CO2 sink not because the absortion of free CO2 (that causes acidification and coral reef death), but because the deposition of algae structures and bodies in the ocean floor.

  10. manuel disse:

    With my kindest regards,I should like to put to your consideration what can be read through “Ocean CO2 absorption pumps” in Google.
    Renewing my compliments,have a very good health. Excuse me,also,the retard of comment,but only now I noted your remark.

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