More than one-third of the greenhouse gases emitted into the atmosphere stem from agriculture and forestry. One of the current concerns is to find ways of managing agriculture differently in order to increase the level of carbon storage in soils and limit emission of gases that contribute to global atmospheric warming.
Photosynthesis ensures that plants assimilate carbon dioxide, in the form of plant carbon, part of which (in 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 depend as much on crop practices as on the soil characteristics. However, some agricultural practices (such as fertilizing and irrigation) favour emission of other greenhouse gases, such as methane and nitrous oxide. Alternative plantation management methods often recommended include the omission of ploughing and cultivation under plant cover. IRD researchers are working on quantitative field assessment of different management alternatives for agriculture and forestry in tropical areas. In Brazil, they have been working with local partners (1) and have brought to evidence the advantages of changing over from traditional methods of sugarcane harvesting involving burning to practices that omit burning.
In Brazil, sugarcane plantations occupy nearly 5 million hectares and produce 10 to 15 tonnes (dry weight) of leaves per hectare per year. Traditional harvesting is a manual method and is carried out after burning of the uncut cane. Burning of the leaves immediately changes the plant carbon into carbon dioxide and methane, which add to existing atmospheric concentrations. It also leads to emissions of nitrous oxide, which comes from part of the plant nitrogen. Methane and nitrous oxide have high potential for contributing to global warming, respectively 20 and 300 times higher than that of carbon dioxide. Moreover, plantation burning liberates potentially toxic, polluting carbon-rich ash and, owing to the elimination of leaf litter, favours soil erosion. An alternative to this system is the non-burning method, but this practice demands mechanization of harvesting (2). In this case, the leaves are left lying as a mulch on the ground. Decomposition releases most of their components (80 to 90%) as carbon dioxide into the atmosphere during the year that follows. The remainder (10 to 20%) can accumulate as litter or become incorporated in the first few centimetres of soil, in this way increasing the amount of carbon stored.
Marie Guillaume | alfa
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