Increasing the carbon in the soil

We sometimes forget that our existence and that of all living beings depend on a thin 20 to 30 cm layer of topsoil in which plants find most of their food. In this layer, soil fertility is due to the type of rock under the soil, but even more to the quantity and type of organic matter it contains.

The main component of organic matter is carbon (approximately 58%). The soil is in fact the largest carbon pool on the planet, with 615 billion tonnes in the first 20 cm and 2,344 billion tonnes up to a depth of 3 metres. This quantity dropped significantly in the 20th century owing to intensification of agriculture, generalisation of deep ploughing in developed countries, transformation of hundreds of millions of hectares of meadow to cultivated land and finally deforestation. In the Paris basin in France, the carbon content of loam soils planted with field crops that do not receive organic fertiliser has dropped by 60% in the space of 50 years, from 1.7% to 0.7%. The carbon lost by soil is released mainly in the form of CO2, one of the products of organic matter mineralisation. Throughout the 20th century, agriculture therefore contributed significantly to CO2 emissions by releasing part of the carbon accumulated in the soil for millions of years into the atmosphere. This process is continuing, mainly due to deforestation, but also due to a continued decrease of organic matter in agricultural soil. In France, the forest area is increasing and made it possible, in the period from 1990 to 2004, to store 0.7 million tonnes of carbon a year in forest soil, but during the same period agricultural soil lost some 6 million tonnes of carbon a year, i.e. 9 times more!


The three self-evident truths concerning the ecosystem
  • First truth: carbon is a major element that structures living matter. On Earth, ecosystems and the associated biodiversity depend on the sun which provides inexhaustible and free energy.
  • Second truth: in nature, a few keys govern functioning of the elements. The main idea is summarised in the famous words of Lavoisier (1789): “nothing is lost, nothing is created, everything is transformed”!
  • Third truth: the ecosystem works as a cycle built on perfect balance between 3 complementary functions: production, consumption and recycling.

The 3 fundamental functions of the ecosystem.

Source: NCAT Agriculture Specialist, September 2001-production consommation recyclage.bmp

In nature, the flow of matter occurs in a cycle, which is the only way to generate sustainable development of ecosystems and to reach balance between production of plant biomass, consumption by fauna, and recycling by living communities in the soil to obtain absorbable minerals (figure above). The soil contains significant biodiversity which represents a genuine recycling entity.

Another key to how nature works is permanent soil cover which makes it possible to maximise plant production and CO2 sequestration by photosynthesis. >>> Plant cover

A third key is essential for maximum production of biomass observed in ecosystems. For the natural cycle to work optimally, the soil must never be bare or tilled. The soil is THE home that hosts biodiversity for biomass and organic waste degradation, which allows temporary storage of a very large quantity of carbon. >>> Promoting no-till farming

By bringing together these three essential principles, the ecosystem is capable of sustainably sequestering carbon in the natural cycle: production - consumption - recycling.


Agricultural carbon sinks, a model ecosystem

As plant biomass producers, farmers are constantly interacting with nature. To build sustainable agriculture, the onus is therefore on them to produce as much biomass as possible, to manage it in the best possible way in the consumption phase, to ensure optimal recycling and to store as much carbon as possible. Implementing the principles and cycles of the ecosystem in agriculture will ensure sustainability of environmental functions linked to biodiversity development.

Enrichment and storage of organic matter in the soil depends strongly on carbon inputs and outputs, and on the duration of its retention in the soil in organic form (INRA 2002).

By adopting the natural ecosystem model to create sustainable agriculture, it is clear that creating an agricultural carbon sink will make it possible in the future to produce more and better, with less input, while developing biodiversity. This carbon sink agriculture makes it possible to reduce all problems related to economic competitiveness and environmental protection.

The Common Agricultural Policy (CAP) is intended to allow creation and financing of agricultural carbon sinks. Thanks to the significant biodiversity generated throughout the sustainability cycle, they will provide environmental services.


Farming techniques conducive to carbon sinks

Some techniques adopted in agriculture are entirely capable of improving the “environmental” situation of territories.

It is today established that no-till farming increases carbon sequestration in soils (Rattan Lal, 2001) provided that production and return of crop residues is maximised by permanent soil cover.

Soil is not merely a medium for plant production. Techniques using tillage are incapable of copying the functioning of the ecosystem (see figure 2) as they partly destroy habitats and the associated biodiversity (populations). Tillage is probably the main explanation for the environmental degradation spiral.

Figure: The spiral of environmental degradation due to tillage.

Source: K. Schreiber, 2005, Results measured in the Maure de Bretagne comparative field (Ille-et-Vilaine)

Direct-seeding mulch-based cropping systems

Direct seeding mulch-based systems make it possible to produce two crops a year on the same plot: while the first crop is used for its food purposes, the second crop can be used either to rebuild a carbon capital in the soil, or be used for energy purposes. This makes it possible to double the solar energy captured per surface unit and carbon yield through photosynthesis, compared to the conventional approach.

The surface section (straw) of the second crop is exported to produce energy by methanation. This process produces 50 % organic waste that can be used as fertilisers.

Solar energy is used throughout the year to produce biomass, with over double the yield of a conventional system, i.e. 12.5 tonnes of carbon per hectare. This system saves fossil energy, optimises solar energy, produces renewable energy as well as organic fertilisation and fertility while increasing below-ground biomass.

For the same level of food production, it feeds the soil approximately 3 times more than with conventional agriculture and ensures fertility.


Agroforestry, a promising approach

Trees store carbon in biomass that can be mobilised during off-peak periods without competing with crops. The efficiency of farming systems is increased.

Food production is maintained and soil fertility is increased by a return to the soil of some 8 t of C/hectare/year. The combination of horizontality (soil cover throughout the year) and of verticality (bringing in trees with a density of 50 stems/hectare) makes it possible to optimise photosynthesis and thereby to mobilise 16.5 tonnes of carbon per hectare per year. Soils are improved and over one tonne of carbon is stored each year in the wood, to be used as timber, for example. Renewable energy production is increased by energy wood production.


The concept of agroforestry appeared in the scientific community recently, in the 1970s. In fact, the practice dates back several centuries, particularly in tropical countries. Specialists provide the following definition: “agroforestry is a system of land use where trees and crops or animals are used together (simultaneously or sequentially) to obtain products or services useful to man”. The idea is therefore to combine trees with annual crops or meadows, at the same time or alternately, in the framework of rotation.

In temperate countries, although there are many examples of agroforestry involving combinations of fruit trees and annual crops, agroforestry combining forest trees and crops is not common. Scientists are now showing increasing interest, in particular in the United States and more recently in France where INRA researchers launched experiments in the framework of the European SAFE project (Agroforestry Systems for European Farms). One of the types of agroforestry tested by INRA was a combination of cereals and poplar trees. Poplar trees are planted in rows that are 20 metres apart and wheat is grown between the rows. A significant impact on the wheat yield is observed only after about twenty years (approximately -10% around the 20th year), but the lower wheat yield is more than compensated by wood production.

Regarding carbon sequestration, we have few figures given that these experiments have not yet given rise to many measurements. Carbon is sequestered both in the soil and in the wood of trees. According to INRA agroforestry specialist C. Dupraz, an agroforestry meadow can sequester 2.7 tonnes of carbon/hectare/year versus only 1 tonne at most for a meadow without trees. In agroforestry, the production of a hectare is equal, depending on the type of combination, to that of 1.2 to 1.6 hectares on which cereals (or other crops) and trees grow separately. Specialists consider that in Europe 90 million hectares could be used for agroforestry practices. In France, if farmers used agroforestry on 2 million hectares (approximately 10% of farmland), the carbon sequestered would reduce CO2 emissions by 10 to 12 million tonnes each year.

Carbon sinks, a climate challenge

The objective of the European Union is to reduce greenhouse gas emissions by 20% by 2020 (compared to emission levels measured in 1990). France will have to make a significant effort. Its total greenhouse gas emissions represent approximately 557 Mt C-CO2 over the period from 2008 to 201 24. Agriculture represents 20% of emissions. The objective is to divide them by 4. The “Millennium Ecosystem Assessment” undertaken by the UN in 2003 and taken up by the FAO In 20076 shows that all the environmental services claimed by society (quality water and food, fibres, materials, renewable energy, climate regulation, water regulation, pollination, landscapes…) are based on agriculture that improves soil fertility, as a carbon sink.

Only direct-seeding techniques combined with dual crop farming and agroforestry offer the potential to produce more and better, without depleting resources. These experiences in the field have proved their worth and can be generalised on a large scale.

Three levels of complementary action make it possible to tackle global warming:

  • Saving fossil energy: reduction of tillage or zero till. Direct seeding divides fuel consumption by 2 and doubles harvests.
  • Sequestering carbon in soils and biomass. Direct seeding has a sequestration potential of 1t C/hectare/year. In agroforestry, 50 trees per hectare store 2 tC/hectare per year.
  • Producing renewable energy which replaces fossil energies (coal, oil, gas).


The indicators linked to these BMPs are the following:

  • Organic matter level
  • Soil cover
  • Biological activity
  • Biodiversity area
  • Yield/hectare


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