Carbon storage in soils responds to a double emergency: increasing agricultural productivity and combating climate change. An annual growth rate of 0.4% of carbon stocks in the first 30-40 centimeters of soil would significantly reduce the concentration of CO2 in the atmosphere linked to human activities. It was with the aim of transferring more carbon to soil that the “4 per 1000” initiative was launched by France on December 1, 2015, and today brings together many actors around the world. Find out what are these storage processes and what can be the benefits in the fight against CO2 emissions, and therefore in improving agricultural productivity.
The soil, a place for storing carbon… organic
Sequestered as organic matter, the amount of carbon in the soil is two to three times that of the atmosphere. The first meter deep in global soils stores between 1,500 and 2,400 billion tonnes of organic carbon. This represents an even greater reservoir of organic carbon than all the biomass of terrestrial plants! The use of these organic carbon stocks generates CO2 flows and has repercussions on climate change. The challenge is therefore to limit these losses linked to the upturn of land and to increase stocks by implementing virtuous agricultural practices.
As s concrete illustration, in 2013, the European Union made it compulsory for member states to set up an accounting of their greenhouse gas emissions, by integrating variations in soil carbon stocks. In France, 3 to 4 billion tonnes of carbon are stored in the first 30 centimeters of soil, but very differently depending on the land use: if a vineyard soil stores 30 tonnes of organic carbon per hectare, a permanent altitude meadow stores more than 80 tonnes per hectare, while by definition, the artificialisation of soils presents negligible organic carbon storage.
How to ensure additional carbon storage in soils?
No more storage agricultural practice for forests has been identified at present, so the challenge is to maintain the existing stock and the practices to maintain it.
For a moderate cost, two practices would make it possible to achieve additional storage of 12% of the potential in France in permanent meadows:
- Practice eco-fertilization (moderate and complete fertilization)
- Extend pastures rather than mow them, thereby favoring the return to the soil of residues and excrement.
The soils with the lowest organic carbon content are generally in arable crops. It is therefore around these farms that the greatest potential for organic carbon storage lies, through several practices:
- Fight against bare soil by setting up plant covers and intercropping.
- Minimize bare soil and tillage
- Associate legumes with rotations
- Practice eco-fertilization aimed at maximum root development (allow the development of populations of microorganisms)
It is by practicing this set of measures within the farm that the soils will increase the rate of storage of organic carbon.
What if carbon storage in soils increases productivity?
One of the clearly identified causes of declining yields in agricultural soils over the past 20 years has been the decrease in soil organic matter levels.
Accompanied by virtuous practices, the storage of carbon in the soil in the form of organic matter (restitution, intercropping, development of root systems, etc.) will induce in the medium and long term a new biodynamic of organic matter in the soil.
Indeed, the maintenance of the quantity of organic matter is essential to the structure of the soils and their stability against erosion. They play the role of “glue” and allow the aggregation of mineral particles (clays) within the clay-humic complex and they serve as food for living organisms (bacteria, fungi, earthworms …) whose activity is favorable to soil structure. The benefits are therefore both environmental and agronomic.
The crop roots benefit from better aeration of the soil, and the infiltration of water is favored, thus avoiding the phenomena of runoff or stagnation (asphyxiation).
A well-developed root system ensures thet a plant is more resistant to biotic and abiotic stress and therefore more productive.