Along with the oceans and forests, soils are one of the planet's main carbon reservoirs. In the 20th century, carbon stocks fell dramatically due to deforestation, intensive farming and the associated poor cultivation practices. Consequently, large amounts of carbon have been emitted into the atmosphere in the form of CO2 contributing to global warming. IRD researchers and their partners have just published a summary in the Global Change Biology journal on soil organic carbon stocks changes in Amazonia.
Deforestation: a source of carbon emissions
Deforestation is a major contributor to greenhouse gas emissions, particularly carbon dioxide (CO2). It contributes in two different ways: through the clearing and burning of the trees, and through the soils which are stripped and then cultivated. Indeed, these soils then release, in the form of CO2, the carbon they previously stored in the form of organic matter.
This soil response after deforestation is extremely diverse. In the current climate change context, it is crucial that we understand and characterise this response, particularly in the Brazilian Amazon where, up to now, the work carried out on this subject had not been compiled.
A global view of the Brazilian Amazon
IRD researchers and their partners have just published a summary in the Global Change Biology journal, a meta-analysis of carbon stock changes in the region's soils. To complete this analysis, they closely examined around twenty studies conducted from 1976 on cattle pasture land and soya or maize fields that have replaced the forest. They then compared the organic carbon quantities measured in these deforested soils with those recorded when they were initially covered with forest.
Soil carbon stocks fall when land is cultivated
Unsurprisingly, the French-Brazilian research team shows that replacing the forest with annual field crops like maize and soya leads to an average 8.5% drop in the soil's carbon stocks. This phenomenon can be explained by the low quantities of organic matter returned to the soil without forest cover, as well as cultivation practices which favour carbon losses.
However, in pasture land, the quantity of organic carbon in the soil has slightly increased since the forest's disappearance. In fact, the significant root activity of the grasses improves the soils' carbon storage. Pedologists thus observe an average 11% increase in this element in meadows that are not overgrazed.
Pasture land: a moderate restoration potential
However, researchers expected much higher values in pasture land, which are thought to offer significant carbon sequestration potential. Furthermore, the increase in carbon quantities from the grasses in pasture land reaches a threshold after around twenty years. Therefore, it certainly does not offset global greenhouse gas emissions from deforestation.
Lastly, this summary reveals that, contrary to what is observed elsewhere in the world, rainfall amounts have no impact on the carbon storage capacity of Amazonian soils.
Scientists are now exploring the influence of different land management methods such as overgrazing, labour and alternative farming systems like agroforestry on carbon sequestration in Brazilian Amazon soils.
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