Soil as a carbon sink for climate

Ademe, the French Environment and Energy Management Agency, has stated: "Agricultural land and forests currently sequester 4 to 5 gigatonnes of carbon, more than two-thirds of which is in the soil. Any variation in this stock has an impact on national greenhouse gas (GHG) emissions" and therefore contributes to the fight against climate change.

Soils: carbon sinks

Soils play an important role in the carbon cycle. Plants capture carbon from the atmosphere (CO2) and release oxygen (O2) through photosynthesis. Carbon is stored in the soil when plants are rotting. Living organisms in the soil (fungi, bacteria, earthworms) transform them into organic matter. The carbon is then trapped in the soil, which contributes to the absorption of atmospheric CO2. The carbon trapped in agricultural and forest soils enables plant growth and better adaptation to climate change.

As the soil acts as a carbon sink, carbon sequestration is possible with the restoration of land use practices: agroforestry, conservation agriculture, use of cover crops, composts and manures. These are agro-ecological techniques that sequester carbon and combat climate change.

Agriculture Sector Initiative: Food Security and Climate “ 4 pour 1000”

The agricultural sector is one of the largest emitters of greenhouse gases and contributes to climate change. However, agriculture can provide solutions thanks to the soil which is a real carbon sink.

The "4 pour 1000" initiative was launched by France in 2015 during COP21 and demonstrates that by storing CO2 in agricultural soils through appropriate practices, agriculture contributes to the fight against climate change and to food security.

The carbon that is trapped in the soil retains water, nitrogen and phosphorus. The latter, by fertilising soils, enables plant growth. The objective of this programme is to increase the stock of carbon in soils by at least 0.4 ‰ per year, in order to help reduce CO2 in the atmosphere. Indeed, by increasing the carbon level in the soil, this would ensure food security and stabilise the amount of CO2 in the atmosphere. The long-term objective is to limit the temperature increase to +2°C, beyond which the consequences of climate change would be significant, according to the IPCC.

How to do it, for whom and at what cost?

In order to implement this initiative, first of all there is the implementation of policy measures that aim to reduce deforestation and the encouragement of agroecological practices. Agroecology is the study of the relationship between agriculture, plants and the environment.

For example, agroecological practices include :

  • Avoiding the stripping of soils to allow carbon to escape
  • Restoring degraded soils, pastures and forests
  • Planting trees and other vegetation
  • Feed the soil with manure and compost
  • Collect water at the foot of plants

These practices could be implemented by farmers and rural residents. Restoration of agricultural soils does not represent a major expense, however, for forest restoration and agroforestry, higher costs are to be expected. Furthermore, with the implementation of these practices, the accumulation of carbon in soils would continue for 20 to 30 years, applicable on a human scale.  
For more information on the "4 for 1000" initiative, here is a link to a video explaining agroecology and the techniques used in french :  

In pursuit of new initiatives: enhanced weathering for land

Research is being carried out to find new ways of sequestering CO2. The improvement of soil weathering through olivine mineralisation has attracted attention.

The extraction and dissipation from the earth's surface of silicates, such as olivine, has been proposed to accelerate this natural sequestration of CO2 ("enhanced weathering"). Agriculture provides an infrastructure for the dissipation of this mineral, however, the rate of weathering and the impacts on the soil and plants are not well understood. Therefore, laboratory experiments are being conducted by researchers to assess olivine weathering in soil and its effects on plant growth and nutrient uptake.

The enhanced weathering by mineralisation of olivine would be used as a fertiliser thanks to the elements it releases into the soil: calcium, magnesium, potassium, nitrogen, etc.

This dissipation on agricultural land, forests and plantations would provide a richer mixture of mineral nutrients and an alternative to certain agricultural amendments such as lime or gypsum. Indeed, according to laboratory research, the dissipation of olivine on land increases the pH and corrects the toxicity of certain soils. The results of the experiments carried out suggest that olivine-based amendments are an effective tool for carbon sequestration and a suitable replacement for lime (C.Dietzen et al., 2018).

Nevertheless, it has been recognised by scientists that olivine releases other minerals such as nickel (Ni) or chromium (Cr) which could have adverse impacts on the terrestrial ecosystem. In agriculture, the concentrations of olivine released on land must be kept low to avoid an imbalance in plant nutrition and to avoid the accumulation of Ni in the soil and crops (ten Berge et al., 2012)

Furthermore, European Directive n°94-27 of 30 June 1994 does not allow the release of these minerals, at a certain threshold depending on the area, being considered carcinogenic for humans. This is why further research must be carried out to determine the ideal quantities to be dissipated on the areas in order to be able to sequester CO2, fertilise the soil and reduce the acidity of the soil by increasing the pH and avoiding secondary effects.