Where Do Biogenic Carbons Come From?

Carbon is the fourth most common element in the universe, behind hydrogen, helium and oxygen. There are some 65.5 trillion metric tonnes of the stuff on planet Earth alone, with the majority of that figure stored in rocks. The rest of it is found in the atmosphere and the ocean, as well as biogenic sources of carbon such as plants, trees, grasses and soil.

The exact volume of carbon in the Earth’s environment does not actually change over time, but simply passes from one region to another. Therefore, it’s not accurate to determine an origin for biogenic carbons – but it is possible to state the feedstocks which are responsible for their emission into the atmosphere, as well as trace their evolution among the natural cycle of carbon on Earth.

Sources of biogenic carbon emissions

Biogenic carbon is stored in organic matter such as trees, plants and grasses, with soil another important storage source. When the former three reach the end of their life cycles and die, their matter decomposes and over time, emits the carbon that they have absorbed throughout their lives back into the atmosphere.

However, these emissions can be accelerated when the organic matter is combusted by humans. This occurs at biomass power stations, as well as through land-clearing and deforestation drives. Even though the carbon is released back into the atmosphere at a faster rate than it was absorbed, the organic matter which is planted to replace it should theoretically absorb the equivalent amount of carbon over time. Of course, this does not happen in the instance of deforestation and land-clearing.

What happens to biogenic carbon after it is emitted into the atmosphere?

Most carbon is capable of persisting in the atmosphere for between 20 and 200 years, but as much as a third of it can endure for millennia. Eventually, however, it will be drawn back down into organic matter via the process of photosynthesis. New plants, trees and grasses will absorb the carbon and send it into their leaves, stems, roots and the surrounding soil.

Inside the plant itself, the carbon is turned into cellulose, which is an important building block for the plant’s development. In the soil, it can remain undisturbed for up to 500 years. This natural ebb and flow of biogenic carbon between organic matter and the atmosphere keeps concentrations of the element stable. As a result, it also acts as something of a thermostat for the planet, ensuring global temperatures do not rise or fall too sharply.

What about non-biogenic carbons?

Other sources of carbon, such as fossil fuels, operate on a different life cycle than biogenic carbons. That’s because these have been assimilated into oil, gas or coal form over millions of years due to extreme meteorological forces like heat and pressure. However, humans have been extracting and combusting fossil fuels as a source of energy for a couple of hundred years, which has greatly advanced our technological capabilities – but which has negatively impacted our air quality and also played havoc with the natural balance of our ecosystem.

That’s because the carbon is emitted back into the atmosphere in a matter of hours, days or weeks, despite the fact that it takes millions of years to become sequestered. Therefore, such a model of energy generation is not sustainable, which is why scientists are greatly interested in pursuing biogenic carbons as a more environmentally-friendly alternative.