Storing carbon in the soil: what agriculture can do
It’s right there, beneath our feet. In our daily lives, we hardly ever notice it, and yet it is nothing less than the largest carbon reservoir in terrestrial ecosystems. This distinction does not belong to forests or the atmosphere, but rather to the soil. There are approximately 2.4 trillion tons of carbon in the top two meters of soil, which is three times more than what is found in the atmosphere.
Rémi Cardinael, CIRAD ; Armwell Shumba, University of Zimbabwe and Vira Leng, University of Montpellier
In an era of climate change and the urgent need to reduce greenhouse gas emissions, the soil’s impressive ability to store carbon gives us pause for thought. While soils alone cannot, of course, drastically reduce the concentrations of greenhouse gases in the atmosphere that are responsible for global warming, they can nevertheless play a substantial role—not only by preserving significant underground carbon stocks but also by restoring degraded land, particularly through certain agricultural practices that allow more carbon to be sequestered underground. Here’s how.
How carbon enters the soil
It all begins with photosynthesis: during this process, plants capture carbon dioxide (CO2) from the atmosphere within chloroplasts, small cellular organelles rich in chlorophyll. TheCO2 combines with water molecules (H2O) using solar energy, thereby producing carbohydrates (carbon-rich molecules) and oxygen (O2). Some of this carbon captured by the plant enters the soil directly through the plant’s roots, both via root exudation and through the renewal of fine roots.
Carbon can also enter the soil when a plant’s dead leaves fall, or when crop residues are left in the field. Once they fall, these carbon-rich dead leaves cover the ground, decompose, are consumed 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 symbiotic relationship with fungi allows them to make the residues more digestible for the termites.
Certain regions and ecosystems contain very large amounts of soil carbon. This is the case, for example, in boreal regions, where enormous amounts of carbon are stored 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 account for the significant stocks observed.
The main challenge for all these carbon-rich ecosystems—such as forests, wetlands, mangroves, and permanent grasslands—is maintaining these carbon stocks rather than increasing them, as this carbon is considered irrecoverable on a human timescale. This requires halting deforestation and the conversion of ecosystems into cropland. 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. Widespread adoption of these practices is one of the goals 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 highlighted, three come up regularly. The first is no-till farming or reduced tillage. This technique involves planting crops without first tilling or plowing the entire field. This practice helps reduce soil erosion, slow the decomposition of organic matter by reducing soil aeration, and preserve soil biodiversity (particularly earthworms).
The second practice promoted is that of maintaining permanent soil cover, either through mulch made from crop residues left in the field or through living cover crops grown between different crops. This soil cover protects the soil from erosion—particularly water erosion—helps sequester carbon, and benefits soil fauna (bacteria, fungi, earthworms, etc.).
The third technique promoted is crop diversification, either through crop rotation or intercropping. This diversification helps limit the spread of pests and plant diseases, while also increasing the productivity of cultivated fields, particularly through the residual effects of previous crops. For example, a legume (peas, beans, peanuts, fava beans, alfalfa, etc.) in the rotation will fix nitrogen from the air and make it available in the soil for the next crop, thereby promoting its growth. Higher crop productivity allows for more carbon to be fixed on the plot, and thus more carbon in the soil, particularly through 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 at increasing soil carbon when used in combination. When 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. It took the scientific community some time to realize this because research initially focused mainly on the top few centimeters of soil, which, under no-till conditions, did indeed have higher carbon content.
However, this was sometimes accompanied by a reduction in soil carbon in the deeper layers compared to tilled systems, where soil carbon is homogenized over a depth of 20 or 30 cm. No-till farming therefore, in some cases, primarily affects the redistribution of carbon within the soil profile, without necessarily leading to a net increase in overall carbon stocks—which is essential when addressing climate change mitigation. A recent synthesis of studies conducted in sub-Saharan Africa suggests that only the combination of the three pillars of conservation agriculture can significantly increase soil carbon stocks, as reduced tillage alone is ineffective.
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What were 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. On average, it takes 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 trials fourteen years ago on cassava-based systems; cassava is grown on nearly 700,000 hectares in the country and is primarily exported to produce flour for animal feed.
By combining no-till farming and direct seeding, permanent soil cover with cover crops, and crop rotation with maize, we observed a significant increase in soil carbon, with carbon accumulation rates of approximately 0.7 to 0.8 tons of carbon per hectare per year to a depth of 40 cm. The region’s hot and humid climate allows for permanent soil cover with highly productive cover crops, including legumes (crotalaria, cowpea) and grasses (millet), grown between cassava and maize crops, on which maize is then sown.
In this way, carbon is sequestered year-round through photosynthesis, and a deep root system develops, allowing carbon stocks to increase beyond the topsoil layers. This additional carbon sequestration in the soil will continue until a new equilibrium is reached within the system. This experiment is designed to be long-term in order 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. Proper soil management requires a long-term approach rather than a piecemeal one.
In Zimbabwe, in a completely different context—with a seven-month dry season and a five-month rainy season—we also sought to assess the long-term effectiveness of these combined practices. To this end, we have access to a trial established ten years ago by our colleagues at the International Maize and Wheat Improvement Center in a low-input system where maize is the primary crop. We were able to measure soil carbon stocks under different practices, both individually and 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 does not achieve much; in fact, it leads to a slight loss of soil carbon compared to conventional tillage. This is explained on this website by the greater soil compaction that occurs when the soil is not tilled, making it difficult for roots to develop. Furthermore, rain penetrates less effectively and runs off the surface, causing water stress in the corn. Ultimately, corn grows much less well in these systems, so there is less carbon input into the soil via the roots, resulting in a loss of soil carbon.
In contrast, no-till fields covered with mulch made from the previous season’s corn crop residues and incorporating crop rotation do increase carbon stocks, although this effect is limited to the surface layer. Nevertheless, a clear increase in carbon stock is observed, as no loss of carbon at depth has been detected.
What are the obstacles to the development of these practices?
While these results are promising, these practices are not always easy to implement. In Zimbabwe, for example, a major constraint has emerged. Farming systems there are low-input mixed crop-livestock systems (with little chemical fertilizer and little or no mechanization). At harvest time, only the corn cobs are harvested by hand, and the corn stalks remain standing in the field. These stalks serve as feed for livestock during the dry season, when cows come to graze directly in the fields after roaming through forests and communal areas during the wet season.
There is therefore competition over the use of corn stalks—whether to feed livestock or to cover the ground. Some farmers erect fences to prevent livestock from eating the stalks during the dry season, which comes at a cost. Others harvest the stalks and store them in elevated areas, out of reach of the animals, and bring the mulch down as the rainy season approaches. This requires significant organization, as well as additional time and energy. In both cases, an alternative feed source must also be found for the livestock.
In these areas, as in others, the value of these practices for farmers does not lie in carbon sequestration in the soil or its impact on climate change mitigation. These techniques are primarily favored for their positive impact on soil fertility and the resulting crop productivity, by 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 top priority for farmers in the Global South, who are among those most affected by climate change.
This article is part of a project in collaboration between 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 & Melinda Gates Foundation. AFP and The Conversation France maintained their editorial independence at every stage of the project.
Rémi Cardinael, Agricultural Researcher, CIRAD; Armwell Shumba, Agronomy Researcher, University of Zimbabwe and Vira Leng, PhD student, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.