Carbon cycle and GHGs
Séquestration, réduction des émissions, protoxyde d'azote et méthane, ...
The carbon atom is one of the most abundant elements on earth, and is a basic component of all known life forms.
Understanding its biogeochemical cycle allows us to better understand its importance in agriculture, and to better appreciate the environmental and climatic issues for which it is regularly mentioned.
The carbon cycle: generalities and its application to agriculture
On the scale of an agricultural plot, two 'natural' sources of carbon can be considered: atmospheric carbon absorbed by plants via 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 or 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 by harvesting, and returned to the soil through the crop residues where it will be partially transformed into CO2 again through the action of micro-organisms. In cropping systems that include a livestock workshop, a significant part of the crops can be used to feed the herd. In this configuration, part of the plant carbon is returned to the plot via manure in particular. On the other hand, there are still two significant outputs from the system: atmospheric losses (CO2, CH4, etc.) and animal production (meat, milk, etc.[1].[2] https://publications.gc.ca/Collection-R/LoPBdP/BP/prb0038-f.htm.
In short, whether we describe it on a plot or global scale, carbon is an element that is constantly being recycled. The specificity of an agricultural plot approach is that it can seek 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[3] In the form of methane (CH4 ) and carbon dioxide (CO2), carbon plays an important role in enhancing the greenhouse effect. However, other gases are responsible for this process, such asnitrous oxide (NO2), water vapour (H2O ) and chlorofluorocarbons (CFCs)[4]
Agricultural activities can cause some of these gases to escape into the atmosphere (hydrocarbon combustion, ruminant fermentation, denitrification etc.). In fact, more and more practices and incentives (such as the national low-carbon strategy) are being put in place. This environmental issue has thus become social, political and progressively economic with the development of agricultural sectors and labels that can grant specific funding. One example is the low carbon label.
Agriculture and carbon sequestration
Although agriculture is sometimes singled out for its greenhouse gas emissions, it is in fact, along with forest management, the only real solution for storing atmospheric carbon in the soil. Indeed, if we take the example of the plot 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 iscalled "carbon sequestration" and in terms of surface area and capacity, agricultural and forestry areas are the most powerful lever for achieving carbon neutrality on a global scale. To promote carbon sequestration and reduce a farm's emissions, there are a number of technical and agronomic levers to be mobilised, the main themes of which are listed here:
- Maintain plant cover for as long as possible during the season, e.g. conservation agriculture, direct seeding, introduction of intercrops, perennials or grasslands in rotations, etc.
- Rationalise hydrocarbon consumption; e.g. simplified cultivation techniques, reduction of mechanised interventions, conscientious maintenance and use of motorised equipment within optimal parameters, optimising journeys etc.
- Establish or maintain landscaping: hedges, flowery and grassy strips, stream edges, embankments, agroforestry, etc.
- Preserve soil organic matter; for example: reduce the frequency of ploughing, prefer organic fertilisers to mineral ones, return crop residues etc.
- Favour locally produced inputs and feeds; 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. It is sometimes singled out for its emissions[5][6] it is nevertheless one of the main assets for mitigating global warming. In fact, more and more considerations, studies and projects are exploring the possibilities of a carbon-capturing agriculture, with, for example, the 4 for 1000 initiative in France.
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References
- ↑ )
- ↑ Forge, CANADA Parliamentary Research Branch, Science and Technology Division, Carbon Sequestration in Agricultural Soils. 2001.
- ↑ ." Environmental Protection Agency, 2002, In brief: The U.S. greenhouse gas inventory. Washington D.C., United States Environmental Protection Agency. Adapted from O. Lee et al, Conceptions of the Greenhouse Effect and Global Warming among Elementary Students from Diverse Languages and Cultures, 2018. https://www.tandfonline.com/doi/abs/10.5408/1089-9995-55.2.117.
- ↑ . T. Latake et al, The greenhouse effect and its impact on the environment, International Journal of Innovative Research and Creative Technology (IJIRCT), 2015. https://www.researchgate.net/publication/302899977_The_Greenhouse_Effect_and_Its_Impacts_on_Environment.
- ↑ .
- ↑ Massemin, Reporterre, Climat : l'agriculture est la source d'un quart des émissions mondiales de gaz à effet de serre, 2015. https://reporterre.net/Climat-l-agriculture-est-la-source,
