BY DR. JOSE PALU-AY DACUDAO
HOW THEN did carbon dioxide decrease and oxygen increase in the atmosphere to present-day levels? After all, whatever carbon dioxide is taken away from the atmosphere by photosynthesis is promptly returned back by biological decomposition or wildfires.
The answer is that much of organic matter, rich in carbon derived from atmospheric carbon dioxide, did not decompose or burn. Instead, over billions of years, a significant amount got buried underground, and stayed there.
Eventually, under conditions of high temperature and pressure underground, most of the oxygen, and much of the hydrogen, readily transform into volatile water. This process resulted in the carbon-rich fossil fuels petroleum and coal. These got left behind underground. Until the present, most of the early Earth’s carbon dioxide reside underground in carbon-rich compounds although most are not minable, being found too deep or too dilute or both.
From the above fundamental processes, we can derive this basic principle.
The more photosynthesis:
6 CO2 (carbon dioxide taken from the atmosphere in a geological instant of a few decades or less) + 6 H2O (water) → C6H12O6 (organic substance) + 6 O2 (oxygen)
Or the less decomposition:
C6H12O6 (organic substance) + 6 O2 (free molecular oxygen in the atmosphere) → 6 CO2 + 6 H2O (water) [in a geological instant, of days to a few years]
The more carbon dioxide fixation (away from the atmosphere).
We can then derive certain conclusions.
The harder to decompose an organic substance, the more it can act as a carbon sink. Most of organic matter in the biosphere are cellulose, hemicellulose, and lignin. Lignin is by far the most difficult to biologically decompose. Why? Lignin is composed of cross-linked polymers of aromatic phenols (carbon-chains arranged in a ring), stacked up on top of each other. This makes lignin hydrophobic and protects its innards from water borne enzymes (meaning all enzymes). A microbe needs to introduce enzymes dissolved in water into a substance in order to decompose it. (That’s why wood stays as wood for a long time, compared to leaves made of cellulose and hemicellulose, which decompose quickly.)
In addition, lignin is extraordinarily rich in carbon. Lignin is made up (by atoms) of almost equal parts of carbon and hydrogen, with oxygen just one third of the carbon. Note that glucose only has equal parts of carbon and oxygen atoms and with hydrogen atoms double the number of carbon atoms.
People that crave to remove carbon dioxide from the atmosphere may also cheer the fact that about 30% of the biomass on our continents is hard-to-decompose lignin. Every woody tree and shrub makes it.
Lignin though can be decomposed by certain bacteria and fungi, although that may take quite a long time. On the other hand, buried lignin, away from the oxygen needed in aerobic decomposition, tends to stay in the earth for hundreds, thousands, or if buried sufficiently deep enough, for millions of years. It turns into carbon-rich peat, coal, and ultimately into elemental carbon in the form of graphite, as water is squeezed out of it. This is a form of carbonization.
C6H12O6 (organic compound) → 6C + 6H2O
Note that coal and graphite are practically invulnerable to biological decomposition. Can they burn in the natural setting? Coal may rarely burn in underground coal fires, but graphite is highly resistant to combustion (unless a nearby volcano blows up).
Note: There is this particularly hare-brained idea of physically sequestering carbon dioxide produced in factories in massive underground tanks. The process alone of doing this requires much energy, likely to be gotten from the combustion of fossil fuels which release more carbon dioxide, thus defeating its purpose. Moreover, carbon dioxide is an industrial gas, used in making dry ice, carbonated drinks, methanol, urea, other chemicals, and infused into greenhouses in order to promote plant growth. These industries will seek that carbon dioxide elsewhere if a country has a principle of tanking all its supply of carbon dioxide underground.
As can be seen from the equations above, dumping massive amounts of wood in anoxic sea bottoms will do the same far more cheaply and easily, if one just wants to sequester carbon dioxide away from the atmosphere.
How about specific ecosystems such as forests? Are they carbon sinks?
A mature primary forest that forms a climax community (meaning it maintains its ecosystem without change) probably is a poorer carbon sink than ecosystems that actively produce lignin in massive amounts such as secondary forests. Growing trees and shrubs are ravenous carbon dioxide gulpers as they transform it into parts of their growing woody bodies.
One of the most popular legends of our present-day milieu of mass and social media, and advertisements is that deforestation always results in net carbon dioxide emission. There is no black or white answer. It’s conditional. If you cut down a primary forest and sequester its wood in houses and other man-made structures (or if you just dump them in anoxic sea bottoms), the carbon dioxide it fixed over the centuries will remain fixed. But if one burns that wood or expose them in environment wherein it can be decomposed, it releases its sequestered carbon back into the air as carbon dioxide.
Here’s another important note. Secondary growth in the form of fast-growing, lignin-producing shrubs and trees, replaces those primary cut-down forests quite readily in areas with adequate soil and rain. In a tropical country, this is very, very obvious. Areas in the hinterlands that got wiped clean of their primary forests start turning green in weeks. In several months to a few years, dense woody growth covers them. The woody growth often is even denser than in primary forests. So contrary to popular legend, a deforested area as long as it retains soil and catches water, may be more massive carbon sinks than mature forest climax communities. (The usual exception is the stripped-mined area, which is stripped away of the soil needed for plant growth, in which case natural reforestation will take decades.)
I am certainly NOT advocating for cutting down primary forests. But preserving them for being massive carbon dioxide net fixers is not a reason for their preservation, because they are not. Their importance is due to their being a genuine part of nature, and they represent biodiversity at its best. (Tropical rainforests are the most bio-diversified ecologies in the biosphere, with more species than other regions on the planet.) That is why they should be preserved.
What about peat bogs, marshes, and other anoxic ecosystems? The answer is obvious from the above equations. They are genuine carbon sinks. Photosynthesis, taking away carbon dioxide from the air, adds to their organic matter, which sinks down, away from atmospheric oxygen. Thus, decomposition back to carbon dioxide proceeds very slowly, or not at all.
The same is more true for anoxic sea bottoms.
How about the traditional practice of burning the remains of sugar cane (or other woody material) after a harvest? Traditionally, it is known NOT burning will result in crop failure. (Never mind the Philippine’s clean-air act, which needs to be amended, or just abolished and cleaned out, leaving anti-pollution laws to local government units that know the local conditions better.) It happens that burning the harvested field results in charring of the woodier parts of the mature sugar cane. This produces charcoal.
In case of the sugar cane fields of Negros, ever wondered why some of them have been growing cane for more than a hundred years, and yet remain well-conditioned? Once plowed into the ground, char (carbon) results in a carbon-rich soil, the same as the very fertile and famous tera preta of the Amazon. Soil rich in elemental carbon retains water, minerals, and other nutrients far better than soils that are not, and prevents acidification.
It has turned out that the slash and burn system, which by legend is detrimental to the ecosystem, that native Amazonians have employed has actually resulted in one of the richest soil in the world, wherein plants grow faster and denser.
Moreover, according to the formula describing charring, which is also a form of carbonization:
C6H12O6 (organic compound) → 6C + 6H2O
The carbon organic material becomes fixed as elemental carbon for a very long time.
The basic principle we can learn from the above that any charring and the incorporation of the char into the ground results in richer soils. Apart from sequestering carbon into the ground and away from the atmosphere. Tera preta type soils (or the process that makes them) are genuine carbon sinks./PN
