The biogenic carbon cycle is a natural process that describes how carbon is recycled in the environment between plants, animals (ruminants), and the soil. The bulk of ruminant feeds are plant materials (e.g. forages) which are high in cellulose. Cellulose is a structural carbohydrate found in all plant tissues, and it’s the major component of plant cell walls which gives them rigidity. It is made up of multiple glucose units that are joined together by β-1,4-glycosidic bonds which makes it resistant to breakdown by other digestive enzymes, except cellulase. Humans and other monogastric animals are unable to digest cellulose because of the absence of cellulase in their gut, but ruminants can break down cellulose through the action of cellulolytic microbes in the rumen, and use the energy created for their physiological processes, and for meat, milk, and wool production. Most of the energy generated in ruminants comes from cellulose fermentation in the rumen, and a benefit of this is that this carbon source does not compete for human edible food but generates a very high-quality food source for humans.
Cellulose is formed in plants when they generate energy through the process of photosynthesis. During this process, they absorb carbon dioxide (CO2) from the atmosphere, use the carbon to form cellulose which is stored in their roots, leaves, and stems, which is a means by which they fix carbon within their tissues, and then release the oxygen (O2) back into the atmosphere. Ruminant animals then feed on these plants, ferment them in their rumen to yield volatile fatty acids which are channeled for various functions in their system, and then use the carbon from the CO2 (a by-product of enteric fermentation of the plant materials) to form methane (CH4), which is released into the atmosphere. The CH4 emitted stays in the atmosphere for about 12 years after which it is converted back to CO2 and water (H2O) through a process known as hydroxyl oxidation. The CO2 is then reabsorbed by plants during photosynthesis to form cellulose, and so the cycle continues.
The global carbon cycle illustrates the movement of carbon between all the sources from which it is generated (fossil fuels, oceans, the atmosphere, humans, etc.), and how it is stored, while the biogenic carbon cycle focuses on the recycling of carbon from biological sources (e.g., plants). The rate/amount of carbon released from other sources like fossil fuels outpaces the ability of other systems in the global cycle to sequester the same equivalent of carbon, but the biogenic carbon cycle is a faster process with a shorter time span, hence, it has a better potential for carbon sequestration and greenhouse gas emission mitigation. The biogenic carbon cycle also illustrates that the CH4 emitted by cows and other ruminant animals does not necessarily add new carbon into the atmosphere, but it is recycled due to the unique ability of ruminants to breakdown cellulose.
One question that arises from this is how biogenic carbon can be accounted for when considering the quantification of greenhouse gas emissions into the atmosphere as part of efforts to address global warming and climate change. A second question: If we can capture more carbon and produce less CH4 because of the mechanism of the biogenic cycle, i.e., if we get very good at minimizing carbon release into the atmosphere in the form of CH4, while our forage crops still absorb CO2 from the atmosphere for cellulose production, is that creating a net negative carbon cycle?
— Gift Omoruyi