No abstract

<p>During the last twenty years, Mayotte island (France, Comoros archipelago) has experienced an explosion of population (+80% between 2002 and 2021, taking the official population into account only) leading to a deforestation and a decline in traditional agricultural practices (<em>mahorais</em> garden) in favor of intensive food crop monocultures (e.g. banana, manioc…). These changes in land use are at the origin of an acceleration of erosion dynamics and an excessive transfer of sediments to water bodies downstream. In addition to land degradation, sediment transfers contribute to the filling of reservoirs devoted to supplying drinking water, to the deterioration of the lagoon and to a general loss of biodiversity.</p> <p>The inertia, trajectory and the sources of this erosion remain poorly documented despite the acceleration of this land use evolution during the last 10 years. In this study, we propose a retro-observation of sedimentary fluxes and sources of sediment based on the analysis of sedimentary archives collected in one of the main reservoirs of the island (<em>Dzoumogné reservoir</em>, 22 ha) along with the development of a sediment tracing approach (associating radionuclide measurements, elemental geochemistry, organic matter and color analyses) in order to reconstruct the evolution of erosion rates and sources of since 2011.</p> <p>The first results reveal the occurrence of two periods of erosion acceleration between 2013 and 2015, and then after 2019. The first increase (+140% of erosion rates, from 3.5 to 8.5 t ha<sup>-1</sup> yr<sup>-1</sup>) occurred during a period of deforestation, which induced an opening of the landscape (e.g. creation of unpaved roads). The second acceleration phase took place during the most recent period of agriculture expansion associated with an intensification of agricultural practices (+115% of erosion rates, from 3 t ha<sup>-1</sup> yr<sup>-1</sup> in 2017 to 6.5 t ha<sup>-1</sup> yr<sup>-1 </sup>in 2020). The sediment accumulated in the reservoir between 2011 and 2021mostly originated from subsurface sources (bank erosion, unpaved roads, gullied soils) and intensive cultivation (e.g., in 2021, 35% of sediment originated from unpaved roads, channel banks, landslides and 27% from intensive cultivation) while forests that are less exposed to erosion only supplied a limited contribution to sediment (<18%). As land use changes are expected to continue in the coming years, understanding the factors controlling the erosion dynamics and the supply of sediment sources are of prime importance in order to implement effective conservation measures and to protect the land and water resources.</p>

International audienc

Esta pesquisa trata sobre a expansão da soja na região amazônica por meio dos grandes produtores de soja. Teve como objetivo descrever a organização do espaço geográfico regional por meio da identificação dos elementos básicos da geopolítica da soja propostos por Becker (2007) : abertura de fronteiras, como a disponibilidade de terra barata e mercados, favorecendo a instalação de atividades produtivas com rapidez, em grande escala e a baixos custos ; a logística caracterizada pelas redes técnicas que viabilizam a integração produtiva-mercantil-financeira ; a integração política que corresponde a alianças estratégicas estabelecidas pelos produtores ; as relações de trabalho do grande produtor com seus funcionários ; e a ciência e tecnologia, responsável pela movimentação de toda a base produtiva. O estudo permitiu entender a evolução dos territórios da soja definida em quatro fases a partir da década de 1980. Estas fases baseiam-se acerca de uma idéia comum de melhoria da situação econômica da soja ; de uma ruptura na maneira de pensar o desenvolvimento da agricultura no estado do Mato Grosso e questionamentos sobre o impacto desta ruptura.