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Adapting agricultural practices to reduce the greenhouse effect

November 23, 2004

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.

Comparative and quantitative study of these two management methods, conducted over a period of 3 to 6 years, showed that the adoption of the non-burning method induced in the first years increased storage of carbon in the soils and a reduction of total emissions of oxides of nitrogen and methane. The average quantity of litter produced in one year was estimated at 10.4 tonnes per hectare, which represents about 4.5 tonnes of carbon. Thus, in the first 20 centimetres of soil, up to 1.6 tonnes more carbon are stored during the first four years of cultivation, compared with the traditional practice using burning. Whereas few differences are observed for emissions of methane and nitrous oxide measured on the soil surface, the absence of leaf burning avoids the emission of a significant amount of these gases into the atmosphere.

Globally, carbon storage in soils and the limitation of gas emissions lead to a net annual gain of 1837 kg equivalents of stored and/or non-emitted carbon. In fact, if the whole of the surface area devoted to sugarcane in Brazil was managed using non-burning systems, the annual sequestration of carbon would represent about 15% of the emissions ascribable to the use of fossil fuels in the country.

Furthermore, this harvesting practice appears beneficial for soil fauna activity and diversity. The traditional practices in fact lead to a sharp fall in diversity and faunal biomass from levels in soil that pre-existed the start of sugarcane growing. However, just three years of the non-burning system were sufficient to restore a diversity and a faunal activity equivalent to those of the initial soil. Adoption of the non-burning system in Brazil, which has advantages for both human health and the environment, could therefore provide the country with a means of contributing to the restriction of the greenhouse effect, even of eventually joining the international market for carbon. However, this practice, which involves a change-over from manual harvesting to mechanized methods, implies substantial financial investment and a sizeable loss of jobs.

Marie Guillaume - IRD

(1) For this investigation, the IRD research unit " Séquestration du carbone dans les sols tropicaux " worked, in Brazil (Piracicaba), with the Laboratory of Environmental Biogeochemistry of the CENA (Centro de Energia Nuclear na Agricultura, University of S'£o Paulo).

(2) In the State of S'£o Paulo, responsible for over half of Brazil's sugarcane output, legislation now obliges sugar refineries and distilleries to move over gradually to a non-burning harvest system. This legislation is founded on government decree n°10.547 of 2 May 2000, supplemented with decree n°11.241 of 19 September 2002.



Institut de Recherche pour le Développement, Paris (IRD)