Sustainable Farm Insights: it’s really all about carbon

Some time ago a colleague told me he does not like the word ‘sustainability’. It really means nothing specific and so it means different things to different people. “When you stand back from the concept, it is really all about carbon,” he said. Having thought a bit about it, I guess he is largely right in what he said.

Carbon is contained in virtually everything in life. All living things are potential stores of carbon and when they die and decay the carbon can ultimately be released to the atmosphere in a range of gases, the most common of which tend to be carbon dioxide and methane, which are greenhouse gasses.

While carbon loss to the atmosphere is widely talked about in terms of greenhouse gasses (GHGs) and global warming, carbon is an important element in agriculture because it is very much part of plant growth and so it is useful to understand a little about it.

Carbon – the building block of life

Carbon has been stored in many different formats on earth for millennia. Plant, human and animal tissues contain carbon. Carbon is an important building block in all organic life, but it can be moulded into materials that have vastly different physical characteristics, eg flesh versus graphite or diamonds.

Graphite is found in the lead of a pencil. This 100% carbon product is relatively soft as the carbon is arranged in layers which can easily peel off with friction to leave a mark. So, this form of carbon can be used for writing or sketching.

Diamonds are also a 100% carbon product. They are one of the hardest substances on earth and this gives them uses which range from the decorative and light dispersing functions of a jewel, to a very tough cutting agent on mining machinery.

Both are made almost exclusively out of carbon, but they have different properties due to the way in which the atoms are arranged and bonded. These inorganic forms are stable but the vast majority of carbon in the world is subject to recycling.

Stores of carbon

With so much talk of carbon loss and global warming, it may be useful to consider where carbon is coming from and going to. Carbon is stored in many different things – we have just mentioned diamonds and graphite, but all plants, our bodies, animals, microorganisms etc contain carbon. And when living things die, the fate of that carbon can range from almost immediate decay in the case of a microorganism, to long-term storage in products like timber or bone.

Another common form of carbon storage are hydrocarbons, or fuels. These can occur as oil, gas, coal etc. One universal characteristic of carbon containing products is that they originated from plants at some point in their life history.

While carbon is essential for plant growth, we do not need to apply it because it comes into the plant as carbon dioxide (CO2) from the air to be transformed into plant growth via photosynthesis. Plant materials then make their way into food, either as plant or animal products. Some of the carbon consumed as food remains in animal and human bodies, while more of it is excreted, and more still is exhaled as carbon dioxide.

We are all part of a huge global cycling and recycling system. Carbon is not made but neither does it vanish. It is always cycling somewhere in our ecosystems.

Humans are stores of carbon -- our bodies being approximately 18% C. As global population grows, so does our capture of carbon. An additional 83m people per year, with an average weight of 75kg, would capture an additional 1.1m tonnes of carbon annually.

But these additional humans also exhale a lot of CO2, they emit heat and they use cloths manufactured from artificial or synthetic materials which are produced from hydrocarbons. A similar observation can often be made for shoes. Choosing natural materials like wool and leather provide a replacement for some of these synthetic materials, with a saving on GHGs.

Natures stores

Soil is a huge store of global carbon, as are the oceans. Think of all the seaweed, fish and other creatures that live there – all containing carbon. Back on land there is a lot of talk and noise about rainforest destruction and the ecosystems they represent. Remember, it is the trees, plus the biodiversity, plus the living organisms that provide that rich store of carbon. If the forest is destroyed the others go too and hence the significant loss.

The soil bank

Soil itself is by far the greatest store of carbon. A study published in Nature in recent years estimated where global carbon stocks are stored, both above and below ground, relative to distance from the equator.

While Figure 1 shows that the rainforests along the equator (0 latitude) are significant carbon sources, it is only when we move north of the 50th parallel that soil carbon stocks really become big, and a proportion of that equates to the soil microbe biomass accompanying that organic matter. Ireland mainly lies between the 52nd and the 55th parallel.

In these regions carbon is stored both in the soil and in the many peatlands across these latitudes. There is a real concern that warming would increase the loss from these regions if soils warm and biological activity increases.

Carbon is also stored in plants, insects, birds etc. Practices such as catch cropping help to continuously sequester carbon. Having plants that flower, also leads to more insect life. And more insect life helps bird numbers etc. The more of all of these living things that we have, the greater is the store of carbon in solid form.

Hydrocarbon compounds are made of two elements – hydrogen and carbon. Some have come about following thousands of years of intervention by nature, others are manufactured by nature in processes such as anaerobic digestion to produce biogas and biomethane.

Methane has been at the centre of much discussion in this country due to the levels emitted by ruminants, but it is an actual hydrocarbon and a fuel. But most of the hydrocarbons we consume are from finite earth-based resources which must, at some point, become limited and so alternatives are essential. It is useful to understand how burning hydrocarbons contributes to CO2 release.

A simple explanation of combustion is a methane molecule combines with two oxygens to give a carbon dioxide plus two water molecules, plus heat. Nothing is created or nothing is lost in a chemical reaction, as shown in the combustion reaction formula in Figure 2.

In this reaction, one carbon, plus four hydrogen, plus four oxygen convert to different substances containing one carbon, plus four hydrogen, plus four oxygen, plus the heat which is the reason for the combustion.

The energy is delivered from the heat, and the combustion drives the internal combustion engine. The design and engineering of the engine converts the combustion to rotation and power.

The combustion process means that the carbon coming out of the ground as fossil fuel ends up in the atmosphere as CO2, hence the implication for global warming. The use of methane (biogas) would act as a recycling of the carbon, rather than adding to it when we extract hydrocarbons from the ground.

Ultimately the stated objective is to replace hydrocarbon use with electric vehicles powered from renewable sources.