Effets des conditions environnementales et des pratiques culturales sur les flux de carbone et d'eau dans les agrosystèmes

Abstract : Agrosystems represent an important proportion of terrestrial ecosystems (more than one third in Europe) and play a key role in sustainable development issues. For instance, they produce green house gasses (GHG) that contribute to climate changes whose feedback on agrosystem functioning and management are still difficult to predict. Therefore, studies on biogeochemical cycle in agrosystems and their GHG production are essential. The main objective of this PhD thesis was the study of cropland carbon and water cycles using 1) micro meteorological measurements of mass and energy fluxes at the soil/vegetation and atmosphere interface and 2) data describing crop dynamics (phenology, biomass, leaf area) for two experimental crop sites located near Toulouse in South West France. This work showed that micro meteorological measurements of fluxes by the eddy covariance method (EC) are well suited to quantify agrosystem carbon and water budgets and to study the main physical and ecophysiological processes driving the different fluxes. From these measurements, carbon and water fluxes were analysed. A methodology to compute GHG budgets at plot scale considering GHG emissions associated with farm operations was developed at our sites and applied to the CarboEurope-IP cropland sites, representing an important range of crop species and management regimes. Annual net CO2 fluxes measurements between the soil/vegetation system and the atmosphere showed that agrosystem act mostly as atmospheric carbon sinks. However, accounting for carbon inputs through organic fertilisation and seeds and outputs through harvest together with vertical CO2 fluxes measurements showed that cropland are rarely carbon sinks and most often act as carbon sources (127 ± 243 g C m-2 y-1 for 41 site-years of the CarboEurope-IP network). On average for these 41 studied site-years, vertical CO2 fluxes represented 37 % of the carbon budget (88 % of the carbon inputs), carbon inputs through organic fertilisation and seeds represented 5 % of the budget (12 % of the carbon inputs) and carbon exportation at harvest represented 58 % of the carbon budget. GHG emissions caused by farm operations represented only 8 % of the annual GHG budget. These results showed that accounting for biospheric CO2 fluxes (88 % of carbon inputs) is fundamental for the evaluation of the annual GHG budgets; otherwise this budget would be strongly overestimated. Water use efficiency (WUE) was addressed through agronomical (biomass production divided by water release by evapotranspiration) and environmental (net ecosystem carbon production divided by water release by evapotranspiration) point of view. These approaches could produce non-consistent results in the case of crops with high yields (i.e. maize used for silaging) and should therefore be carefully considered in the perspective of a sustainable cropland management. Finally, flux measurements were used 1) to test a beta version of the ICASTICS model that will be used to assess processes and estimate the carbon and water net fluxes components for a range of crops, management regimes and climatic conditions. 2) .to parameterise new functions in the SAFY model, so it could calculate both water and carbon fluxes and budgets at spatial resolutions larger than plot scale, in combination with remote sensing data, in a natural resources management perspective
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Earth Sciences. Université Paul Sabatier - Toulouse III, 2009. French


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Submitted on : Friday, January 15, 2010 - 10:59:10 AM
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Pierre Beziat. Effets des conditions environnementales et des pratiques culturales sur les flux de carbone et d'eau dans les agrosystèmes. Earth Sciences. Université Paul Sabatier - Toulouse III, 2009. French. <tel-00447499>

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