Carbon cycle and greenhouse gases

Sequestration, reducing emissions, nitrous oxide and methane, etc.

Carbon cycle and GHGSequestration, reducing emissions, nitrous oxide and methane, etc.

1. Themes
2. Carbon cycle and greenhouse gases

The carbon atom is one of the most abundant elements on earth, and is a basic component of all known forms of life.

Understanding its biogeochemical cycle gives us a better grasp of its importance in agriculture, and a better appreciation of the environmental and climate issues to which it is regularly referred.

The carbon cycle: generalities and agricultural applications

On the scale of an agricultural plot, there are two 'natural' sources of carbon: atmospheric carbon absorbed by plants through their photosynthetic activity, and to a lesser extent carbon from parent rocks, which can be slowly oxidised and released into the soil.

From atmospheric carbon (in the form of CO2), plants produce molecules essential to their growth, such as carbohydrates and cellulose. This energy metabolism gradually generates a certain amount of CO2 through respiration, which is then released back into the atmosphere. At the end of this process, the carbon remaining in the plants is exported through harvests and returned to the soil through crop residues, where it is again partially transformed into CO2 through the action of micro-organisms. In cropping systems that include a livestock unit, a significant proportion of the crops may be used to feed the herd. In this configuration, some of the carbon from the plants is returned to the plot via manure in particular. However, there are still two significant outputs from the system: atmospheric losses (CO2, CH4, etc.) and animal production (meat, milk, etc.. https://publications.gc.ca/Collection-R/LoPBdP/BP/prb0038-f.htm.

In the final analysis, whether we describe it on the scale of a plot of land or on a global scale, carbon is an element that is constantly being recycled. The specificity of a plot-based approach is that it seeks to maintain a balance between inputs and outputs, undesirable losses and storage in the soil.

Greenhouse effect

The greenhouse effect can be defined as "the effect of chemical components of the Earth's atmosphere, called greenhouse gases, that trap outgoing radiation and thus contribute to atmospheric warming In the form of methane (CH4) and carbon dioxide (CO2), carbon plays an important role in increasing the greenhouse effect. However, other gases are also responsible for this process, includingnitrous oxide (NO2), water vapour (H2O ) and chlorofluorocarbons (CFCs)

Agricultural activities can cause some of these gases to escape into the atmosphere (combustion of hydrocarbons, fermentation by ruminants, denitrification, etc.). As a result, more and more practices and incentives (such as the national low-carbon strategy) are being introduced. This environmental issue has therefore become a social, political and, progressively, an economic one, with the development of agricultural sectors and labels that can provide specific funding. One example is the low-carbon label.

Agriculture and carbon sequestration

Although agriculture is sometimes singled out for its greenhouse gas emissions, in reality, along with forest management, it is the only real solution for storing atmospheric carbon in the soil. If we take the example of the plot of land mentioned above, we can see that some of the carbon temporarily "leaves the cycle" when it is stored in the form of organic matter in the soil. This phenomenon is knownas "carbon sequestration", and in terms of both surface area and capacity, agricultural and forestry areas are the most powerful lever for achieving carbon neutrality on a global scale. To encourage carbon sequestration and reduce a farm's emissions, there are a number of technical and agronomic levers to mobilise, the main themes of which are listed here:

• Maintaining plant cover for as long as possible during the growing season; for example: conservation farming, direct seeding, introducing intercrops, perennials or grassland into rotations, etc.
• Rationalise hydrocarbon consumption; for example: simplified cultivation techniques, reduction in mechanised operations, conscientious maintenance and use of motorised equipment within optimum parameters, optimised journeys, etc.
• Establish or maintain landscaping: hedges, flower and grass strips, stream banks, embankments, agroforestry, etc.
• Preserve the soil's organic matter; for example, reduce the frequency of ploughing, use organic fertilisers rather than mineral ones, return crop residues, etc.
• Favour locally produced inputs and feeds; for example: increase the protein autonomy of your herd, reduce dependence on imported inputs, etc.

To sum up

Agriculture is at the heart of the global issue of carbon, the greenhouse effect and climate change. Sometimes singled out for its emissionsE it is nonetheless one of the main assets for mitigating global warming. In fact, more and more studies and projects are exploring the possibilities of carbon-capturing agriculture, such as the 4 for 1000 initiative in France.