Agriculture and Greenhouse

Limit the greenhouse effect through agricultural practices

Agriculture generates about 35% of greenhouse gas emissions. One of the recommended solutions to limit these emissions is to adopt cultivation methods favorable to the storage of carbon in the soil and the reduction of methane and nitrous oxide emissions, the whole constituting the "carbon sequestration". At IRD, researchers quantify the emission and storage of greenhouse gases in cultivated soils in tropical regions. With their local partners (1), they showed the benefits of moving from a sugar cane harvest with burns to a no-burn harvest in Brazil. By proposing viable cultural alternatives, quantitative studies can enable countries with a strong agricultural vocation to participate in limiting the greenhouse effect.

More than a third of the greenhouse gases emitted into the atmosphere come from agricultural and forestry activities. One of the current concerns is to find ways to manage agriculture differently in order to increase carbon storage in soils and to limit emissions of gases that contribute to global warming of the atmosphere. Plants, through photosynthesis, assimilate carbon dioxide in the form of plant carbon, part of which (roots and crop residues) is returned to the soil and stored in a stable form in organic matter. The quantities of carbon stored in the soil come from both cultural practices and the nature of the soil. However, some agricultural practices (fertilization, irrigation, etc.), promote emissions of other greenhouse gases, such as methane and nitrous oxide. Among the proposed management alternatives, the absence of plowing and the cultivation under vegetal cover are often recommended. IRD researchers favor a quantitative field evaluation of agricultural and forestry land management alternatives in tropical regions. In Brazil, they and their local partners (1) highlighted the advantages of switching from a traditional sugar cane harvest to the practice of no-burn.

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In this country, the cultivation of sugar cane covers about 5 million hectares and produces 10 at 15 tons of leaves (dry matter) per hectare per year. The traditional harvest, manual, is done after burning the cane on foot. The burning of leaves immediately transforms plant carbon into carbon dioxide and methane, enriching the atmosphere. It also causes nitrous oxide emissions, derived from a portion of the plant nitrogen. However, methane and nitrous oxide have a high global warming potential, respectively 20 and 300 times greater than that of carbon dioxide. In addition, field burning releases potentially toxic compounds, polluting carbonaceous ash, and, because of the lack of litter, promotes soil erosion. An alternative to this type of land management is non-burning, but this practice requires mechanized harvesting (2). In this case, the leaves are left in mulch on the ground. A major part (80 to 90%) returns, by decomposition, in the form of carbon dioxide in the atmosphere during the following year. The remainder (10 to 20%) can accumulate as litter or be incorporated into the first few centimeters of the soil, thus increasing the carbon stock.

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The comparative and quantitative study of these two management methods, conducted over a period of 3 to 6 years, shows that the adoption of non-burns induces in the first years an increased storage of carbon in soils and a reduction of emissions. total of nitrous oxides and methane. The average amount of litter produced in a year has been estimated at 10,4 ton per hectare, which represents about 4,5 tonnes of carbon. Thus, in the first 20 cm of soil, up to 1,6 tons of additional carbon, compared to the traditional mode with burns, are stored during the first four years of cultivation. While little difference is observed for methane and nitrous oxide emissions measured at the soil surface, the absence of burning of the leaves makes it possible to avoid the emission of a significant amount of these gases into the soil. atmosphere.

Overall, the storage of carbon in the soil and the limitation of gaseous emissions lead to a net annual gain of 1837 kg of carbon equivalents stored and / or not emitted. In fact, if all the areas cultivated for sugar cane in Brazil were managed as non-burned, the annual sequestration of carbon would represent approximately 15% of the emissions attributable to the use of fossil fuels in the country.

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In addition, this mode of harvest appears to be beneficial for the activity and diversity of the soil fauna. Traditional practices induce a sharp decrease in the diversity and biomass of wildlife, compared to the soil that existed before the cultivation of sugar cane. But three years of slash-and-burn management is enough to restore diversity and wildlife activity equal to that of the original soil. The adoption of non-burning in Brazil, which benefits human health and the environment, could therefore enable the country to participate in the limitation of the greenhouse effect, or even to enter the international carbon market at a later stage. However, this practice, which involves moving from manual harvesting to mechanized harvesting, entails a significant financial investment and a significant loss of jobs.
source: Marie Guillaume

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