Trapping carbon in the soil: what agriculture can do
It is right there, beneath our feet. In our daily lives, we hardly notice it, yet it is nothing less than the largest carbon store in terrestrial ecosystems. This distinction does not go to forests or the atmosphere, but to the soil. There are approximately 2,400 billion tons of carbon in the top two meters of soil, three times more than in the atmosphere.
Rémi Cardinael, CIRAD ; Armwell Shumba, University of Zimbabwe and Vira Leng, University of Montpellier
At a time of climate change and the urgent need to reduce greenhouse gas emissions, this impressive capacity of soils to store carbon gives pause for thought. While soils alone cannot drastically reduce the concentrations of greenhouse gases in the atmosphere that are responsible for global warming, they can nevertheless play a substantial role, both by preserving large underground carbon stocks and by restoring degraded land, in particular through certain agricultural practices that allow more carbon to be trapped underground. Here's how.
How carbon enters the soil
It all starts with photosynthesis: during this process, plants fix atmospheric carbon dioxide (CO2) within chloroplasts, small cell organelles rich in chlorophyll. TheCO2 is combined with water molecules (H2O) using solar energy, producing carbohydrates (carbon-rich molecules) and oxygen (O2). Some of the carbon captured by the plant enters the soil directly via the plant roots, both through root exudation and the renewal of fine roots.
Carbon can also enter the soil when a plant's dead leaves fall, or when crop residues are left on the field. Once fallen, these carbon-rich dead leaves cover the soil, decompose, are ingested by bacteria, fungi, or earthworms, and eventually turn into soil organic matter. Animals can also accelerate this process of carbon transfer into the soil, for example, fungus-growing termites, which transport plant residues into their termite mounds where a symbiosis with fungi allows them to make them more assimilable for the fungi.
Some regions and ecosystems have very large soil carbon stocks. This is the case, for example, in boreal regions, where enormous stocks are preserved in the permafrost but are now threatened by global warming. In tropical regions, the high productivity of ecosystems, particularly forests, as well as very deep soils, also explain the significant stocks observed.
The main challenge for all these carbon-rich ecosystems, such as forests, wetlands, mangroves, and permanent grasslands, is to maintain these stocks rather than increase them, as this carbon is considered irrecoverable on a human scale. This requires halting deforestation and the conversion of ecosystems into cultivated land. On average, 25% of soil carbon is lost when forests or wetlands are converted to cropland, and sometimes even more. On agricultural land, certain practices can sequester more carbon in the soil. Promoting their widespread use is one of the objectives of the "4 per 1000" initiative launched at COP21.
Which agricultural practices increase soil carbon stocks?
There are many practices that can increase carbon stocks in agricultural soils, such as agroforestry, cover crops, and organic amendments. Among the solutions often put forward, three come up regularly. The first is no-till farming or reduced tillage. This technique involves sowing crops without first tilling or plowing the entire field. This practice reduces soil erosion, slows down the decomposition of organic matter by reducing soil oxygenation, and preserves biodiversity (particularly earthworms).
The second practice promoted is permanent soil cover, either with mulch from crop residues left on the field or with living plant cover between different crops. This soil cover protects against erosion, particularly water erosion, helps to sequester carbon, and is beneficial to soil fauna (bacteria, fungi, earthworms, etc.).
The third technique promoted is crop diversification, either through rotation or intercropping. This diversification helps limit the development of pests and plant diseases, but also increases the productivity of cultivated plots, particularly thanks to the previous effects of crops. For example, a legume (peas, beans, peanuts, fava beans, alfalfa, etc.) in rotation will fix nitrogen from the air and make it available in the soil for the next crop, thus promoting its growth. Better crop productivity means more carbon is fixed on the plot, and therefore more carbon in the soil, particularly via the roots of the crops.
These three practices correspond to the three pillars of what is known as "conservation agriculture." These practices become truly effective in increasing soil carbon when they are combined. Practiced alone, they sometimes have little or no impact. This is particularly true of no-till farming alone, which can have a positive effect on soil carbon in some contexts but not in others. The scientific community was slow to realize this because research initially focused mainly on the top few inches of soil, which did indeed have a higher carbon content as a result of no-till farming.
However, this was sometimes accompanied by a reduction in soil carbon in the deeper layers compared to tilled systems, where soil carbon is homogenized at a depth of 20 or 30 cm. In some cases, therefore, no-till farming mainly affects the redistribution of carbon in the soil profile, without necessarily leading to a net increase in overall carbon stocks, which is necessary when considering climate change mitigation. A recent synthesis of work carried out in sub-Saharan Africa suggests that only the combination of the three pillars of conservation agriculture can significantly increase soil carbon stocks, as reducing tillage alone is ineffective.
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What are the results in Zimbabwe and Cambodia?
To fully understand the benefits of these three practices when combined, it is crucial to conduct long-term experiments. It takes an average of 5 to 10 years for a significant change in soil carbon stocks to be detected.
In Cambodia, CIRAD and the Cambodian Ministry of Agriculture began experiments fourteen years ago on systems based on cassava, a crop covering nearly 700,000 hectares in the country and mainly intended for export to produce flour for animal feed.
By combining no-till farming and direct seeding, permanent soil cover with cover crops, and crop rotation with corn, we have seen a significant increase in soil carbon, with carbon accumulation rates of around 0.7 to 0.8 tons of carbon per hectare per year to a depth of 40 cm. The region's warm, humid climate allows for permanent soil cover with highly productive cover crops, including legumes (crotalaria, cowpea) and grasses (millet) between cassava and corn crops, on which corn is sown.
In doing so, carbon is fixed throughout the year by photosynthesis, and a very deep root system develops, increasing carbon stocks beyond the topsoil. This additional carbon storage in the soil will continue until a new equilibrium is reached in the system. This trial is intended to be maintained over time to estimate how many decades such a system can store carbon. Once equilibrium is reached, the challenge will then be to preserve these carbon stocks by maintaining good soil management practices. Good soil management requires long-term management rather than sporadic intervention.
In Zimbabwe, in a completely different context, with a seven-month dry season and a five-month rainy season, we also wanted to measure the long-term effectiveness of these combined practices. To do this, we have a trial set up by our colleagues at the International Maize and Wheat Improvement Center ten years ago in a low-input system with maize as the main crop. We were able to measure soil carbon stocks for different practices, either alone or in combination: fields with tillage, fields without tillage, with or without maize crop residue (mulch), and with or without rotation with cowpea, a legume.
Once again, the results show that no-till farming alone cannot achieve much; it even leads to a slight loss of soil carbon compared to tillage. This is explained on this website by the greater compaction of the soil when it is not tilled, which makes it difficult for roots to develop. In addition, rain penetrates less easily and runs off the soil, causing water stress on the corn. Ultimately, corn grows much less well in these systems, so there is less carbon input into the soil by the roots, resulting in a loss of soil carbon.
On the other hand, fields without tillage, with mulch from the previous season's corn crop residues and crop rotation, allow for an increase in carbon stocks, although the effect is limited to the surface layer. However, there is a net increase in carbon stocks, as no carbon loss has been observed at depth.
What obstacles stand in the way of developing these practices?
While these results are promising, these practices are not always easy to implement. In Zimbabwe, for example, there is a major constraint. Agricultural systems are low-input mixed farming systems (little mineral fertilization, little or no mechanization). At harvest time, only the corn ears are harvested, by hand, and the corn stalks remain standing in the field. These are used as feed for livestock during the dry season, when cows graze directly in the fields after roaming in forests and communal areas during the wet season.
There is therefore competition for the use of corn residues, either to feed livestock or to cover the soil. Some farmers install fences to prevent livestock from eating the residues during the dry season, which comes at a cost. Others harvest and store them high up, out of reach of the animals, and bring the mulch down when the wet season approaches. This requires a lot of organization, time, and energy. In both cases, an alternative source of feed must also be found for the livestock.
On these lands, as on others, the benefit of these practices for farmers does not lie in carbon sequestration in the soil and its impact on climate change mitigation. These techniques are particularly popular for their positive impact on soil fertility and crop productivity, reducing the risk of erosion, improving nutrient availability, and enabling adaptation to climate change through, for example, better water conservation. These benefits are crucial and often a priority for farmers in the Global South, who are among those most affected by climate change.
This article is part of a project involving The Conversation France and AFP Audio. It received financial support from the European Journalism Centre as part of the Solutions Journalism Accelerator program supported by the Bill and Melinda Gates Foundation. AFP and The Conversation France have maintained their editorial independence at every stage of the project.
Rémi Cardinael, Agronomy Researcher, CIRAD; Armwell Shumba, Agricultural Researcher, University of Zimbabwe and Vira Leng, Doctoral Student, University of Montpellier
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