Carbon sequestration in French soils
The "4 per 1000" initiative, launched by France at COP-21 in 2015, aims to increase the stock of carbon in global soils by 0.4% per year. This increase would compensate for the annual CO2 emissions caused by human activities. This article is based on a study conducted by INRAE in 2020 entitled « Storing carbon in French soils: What potential regarding the 4 per 1000 target and at what cost? » available in several versions here.
Definition
Carbon sequestration refers to the increase of carbon stock in the soil over time. This process is mainly influenced by two key factors:
- The amount of carbon entering the soil, coming from plant residues, plant roots, and inputs of organic matter.
- The average residence time of this carbon before mineralization, that is to say its decomposition by soil microorganisms and its return to the atmosphere as CO2.
A change in carbon inputs or outputs, if maintained over the long term, will lead to an evolution of the carbon stock until a new equilibrium is reached.
Why sequester carbon?
Carbon sequestration is an essential process to combat climate change and improve soil health.
Reduction of atmospheric CO2
Carbon dioxide (CO2) is the main greenhouse gas responsible for global warming. Carbon sequestration allows trapping atmospheric CO2 in soils, thus reducing its concentration in the atmosphere. The 4 per 1000 target, although ambitious, would offset the annual CO2 emissions caused by human activities.
Improvement of soil health
Carbon is an essential element for soil fertility. By increasing the organic carbon content of soils, their structure, water retention capacity, and nutrients availability, as well as their biological activity, are improved. Carbon-rich soils are more resistant to water stress, contribute to better water filtration, and regulate biogeochemical cycles.
Factors influencing carbon storage
The amount of carbon stored in soils depends on several factors:
- The land use type:
- forest soils: 38% of the total stock
- permanent grasslands: 22%
- arable and temporary grasslands (26.5%) partly due to their large area.
- The soil type and climate: soils in mountainous areas and grasslands have higher carbon stocks due to climate and soil type.
- Agricultural practices: certain practices, such as expanding cover crops and agroforestry, can increase carbon storage.
Without changes in land use, and without modifying agricultural and forestry practices, the evolution of soil carbon stocks is currently estimated, across all land use types, at 2.3 ‰ per year with high uncertainty (-0.2 ‰ to +3.2 ‰ per year). However, this increase is partly offset by land use changes that deplete carbon: soil sealing and plowing of grasslands.
Storage and Additional Storage
It is important to distinguish two concepts:
- Carbon storage: this is the increase in carbon stock in the soil in general, regardless of land use type or agricultural practices used.
- Additional storage: this concept refers to the increase in carbon stock linked to a change in agricultural or forestry practice. It is calculated by comparing the carbon stock under a given practice (for example, agroforestry) with the carbon stock under a reference practice (for example, conventional cropping).
The INRA study focused on the additional carbon storage enabled by adopting more sustainable agricultural and forestry practices.
Agricultural practices promoting carbon storage
Expansion of cover crops
The implementation of cover crops, without biomass export, has a positive effect on carbon storage. Almost the entire area under arable crops is concerned by this scenario. This involves establishing cover crops where they do not currently exist, increasing their frequency in rotations, or extending cover crops already in place.
36% of the total additional storage potential, i.e. +126 kgC/ha/year in the top 30 cm of soil.
Intra-field agroforestry
Planting rows of trees in arable fields can contribute to carbon sequestration both in the soil and in tree biomass.
20% of the total additional storage potential, i.e. +207 kgC/ha/year in the top 30 cm of soil.
Increasing the share of temporary grasslands in crop sequences, by lengthening their duration or by introducing them as a replacement for maize fodder, can also promote carbon sequestration.
13% of the total additional storage potential, i.e. +127 kgC/ha/year
Grass cover between vineyard rows
Significant unit additional storage potential, but low at the national scale.
+182 kgC/ha/year
Permanent grasslands
- Moderate intensification of extensive grasslands, through fertilizer application, can lead to additional biomass production which increases the return of plant residues to the soil and thus carbon sequestration. +176 kgC/ha/year
- Replacing mowing with grazing : Grazing can increase the return of plant residues to the soil compared to mowing, which favors carbon sequestration. +265 kgC/ha/year
Hedge planting
Planting hedges around fields or field blocks of at least 8 ha can also contribute to carbon sequestration.
+17 kgC/ha/year
Addition of new organic resources
The addition of exogenous organic materials, such as composts from organic waste products, can also contribute to carbon sequestration in soils. However, it is important to ensure that the use of these resources complies with regulations and does not raise social acceptability issues.
+57 kgC/ha/year
No-till farming
No-till farming is a cultivation technique that allows direct sowing into untilled soil, without plowing. This practice can contribute to carbon sequestration, especially in the topsoil horizon (0-30 cm), by +60 kgC/ha/year. However, no-till farming has no significant effect on carbon storage when considering the entire soil profile.
The greatest additional storage potential, 86% of the total, lies in arable crops where the current stock is lowest, thanks to 5 practices:
- Implementation of intermediate and cover crops (35% of total potential)
- Introduction and extension of temporary grasslands in crop rotations (13% of total potential)
- Development of agroforestry (19% of total potential)
- Addition of composts or organic waste products, at a negative cost (slight gain for farmers)
- Hedge planting
Cost of storage
Cost by Practice
Practice with negative cost:
Vineyard grass cover : Permanent or winter grass cover between vineyard rows is the only practice with a negative cost (-106 to -56 €/tC). This means that this practice generates additional income for farmers while storing carbon.
Practices with Moderate Cost < 250 €/tC:
- Replacing mowing with grazing (277 €/tC)
- Mobilization of new organic resources (397 €/tC)
- Moderate intensification of permanent grasslands (157 €/tC)
- Expansion of cover crops (180 €/tC)
Practices with High Cost > 300 €/tC:
- Insertion and extension of temporary grasslands (423 €/tC)
- Intra-field agroforestry (302 €/tC)
- Hedge planting (2,322 €/tC)
Conclusions
- Storage potential: The study demonstrates that it is possible to significantly increase carbon storage in French soils. The implementation of nine agricultural and forestry practices identified in the study could allow an annual additional storage of +1.8‰ over all agricultural and forestry areas, i.e. 5.69 MtC/year, or 41% of agricultural carbon emissions. This storage potential is mainly concentrated in arable soils, where additional storage could exceed the 4 per 1000 target.
- Uncertainty on the baseline : The study also highlights the uncertainty regarding the current evolution of carbon stocks under current practices (baseline). Upcoming sampling campaigns of the Soil Quality Measurement Network should help refine these estimates.
- Importance of maintaining practices: The permanence of carbon storage in soils depends on the long-term maintenance of storing practices. It is therefore crucial to implement incentive public policies and raise farmers' awareness of the importance of these practices.
- Costs: Implementing storing practices can incur costs for farmers. However, the study shows that the majority of the additional storage potential can be achieved at a cost below the carbon shadow price in 2030.
- Need for a combined approach: To maximize carbon storage in French soils, it is necessary to adopt a combined approach including:
- Maintaining areas and favorable practices on ecosystems with positive trend storage (forests and permanent grasslands).
- Implementing all storing practices on their full technical area in arable crops and grasslands.
- Stopping land use changes that reduce stocks, such as plowing of grasslands and sealing of agricultural land.
- Co-benefits: The study emphasizes that several of the proposed practices to store carbon in soils provide other benefits, such as improving water quality, protecting biodiversity, and reducing erosion risk.
- Need to reduce emissions: The study reminds that soil carbon storage can only complement efforts to reduce greenhouse gas emissions. Emission reduction remains the primary goal to combat climate change.
In conclusion, the INRA study highlights the significant potential for carbon storage in French soils, while underlining the challenges and uncertainties that remain. It advocates for a global and coherent approach, combining the implementation of more sustainable agricultural practices, protection of ecosystems with high storage potential, and reduction of greenhouse gas emissions.
