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Etude des transferts d'eau et de nitrate dans les sols ferrallitiques de Maré (Nouvelle-Calédonie) : risques de pollution des lentilles d'eau douce

Abstract : The purpose of tbis study was to determine the risk of groundwater pollution from intensified agricultural practices on Maré, in the Loyalty Islands of New Caledonia. To achieve this goal, the water and nitrogen balances were established during three quite different wet seasons, and under different agricultural treatments and soil-cover conditions. Maré is an uplifted coral atoll which is build upon an underlying volcanic structure. Underneath, fresh water lenses are located inside the coral rock and these are floating on the underlying sea water. These lenses supply potable water to the indigenous Kanak people. The Maréan soil is a ferralitic oxisol derived from weathered volcanic ejecta and ash. It is a shallow and very permeable soil, with a saturated conductivity in excess of 100 mm h-1 . This soil possesses a certain degree of anion exchange capacity, that should retard to some degree the leaching of nitrate. The weather of Maré is characterized by a wetter season during which there are frequent tropical storms of high intensity, so the soil water fluxes can be very high. Intensification of agriculture on Maré offers the opportunity to develop economically sustainable enterprises for the Kanak population. However the fragile nature of the ecosystem requires careful land management strategies to ensure that such intensive agriculture is also environmentally sustainable. The impact of land management on the quality of underlying water contained in the fresh water lenses is critical. To calculate drainage and nitrate leaching under the rootzone, non-destructive devices such as Time Domain Reflectometry were used to measure the soil water content, tensiometers for the soil water pressure, and suction cups to extract the soil solution. An infiltrometer provided us with the soil's hydraulic characteristics, and we obtained some hydro-dispersive parameters with soil column experiments in the laboratory. About 100 kgN ha*1 per year was applied to a cornfield, a perennial pasture and a bare soil plot. The results show that pollution risks from fertilizers are significant under this permeable soil in this humid tropical climate. Furthermore, mineralisation of organic nitrogen is high thereby increasing the amount of N-NO3 present in the soil. In 1996, we obtained 200 % losses of nitrate under a fertilized bare soil, as compared to the amount of N-NO3 applied. However, even though the 1996 season was wet compared to 1995, the amount of nitrate leaching on corn was reduced from 130 % (1995) to 48 % (1996) of the N-NO,' applied. In 1996, the nitrate fertilizer was applied in two split applications 50 that the plants were able to take it up more efficiently. In 1995, ail the N-NO3 fertilizer was applied on the day of corn sowing. The grass plot showed that a perennial plant is able to consume most of the applied nitrate throughout the 1996 season, and this decreased the leaching to about 20 % of the NNO3 applied. Despite possessing anion exchange capacity, the high rainfall rates and penneable nature of the soil conspire to render nitrate easily leached. The pollution risk can however be strongly reduced with split applications such that root uptake of nitrate can proceed before the nutrient is leached beyond the rootzone where it can be tenned a pollutant. Sorne fertilization is still considered necessary because these ferralitic oxisols lack a range of nutrients of which nitrogen is one of the most important. Therefore careful application strategies will need to be developed. The soil parameters detennined on the field and in the laboratory allowed us to employa mechanistic model for water and solute transport, and to assess its ability to reproduce and predict our data. The model WAVE (Water and Agrichemicals in the Vadose Environment) was used. The measurements for the three years under different climatic conditions were quite weil reproduced. By the model now validated, it can thus be used to develop efficient and sustainable fertiliser management practices for the farmers on Maré.
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Submitted on : Wednesday, July 18, 2012 - 8:49:06 AM
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Céline Duwig. Etude des transferts d'eau et de nitrate dans les sols ferrallitiques de Maré (Nouvelle-Calédonie) : risques de pollution des lentilles d'eau douce. Environnement et Société. Université Joseph-Fourier - Grenoble I, 1998. Français. ⟨tel-00718715⟩

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