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Impacts agronomiques et environnementaux de l'épandage de vinasse et de cendre de charbon/bagasse sur les terres agricoles de l'Île Maurice

Abstract : Of the 85 000 hectares of arable land in Mauritius, sugar cane occupies some 85% of that area, the remainder is made up mostly of vegetables and fruit trees. To ensure its economic viability and long term sustanainability, the sugar cane industry through a reform program is striving to add value to its by-products by producing ethanol from molasses and by optimizing energy production from bagasse during the harvest season. However these two avenues generate considerable wastes (400 000 T/yr of vinasse during ethanol distillation and 40 000 T/yr of coal/bagasse ash during the production of electricity), which have to be judiciously disposed of with a minimum of risk to the environment. Among the disposal means for these wastes, the application of vinasse and coal/bagasse ash to agricultural land is believed to represent the most sensible economic option from both the agronomic and environmental point of view. In this context before recommending elimination of the wastes through disposal on agricultural lands, a study has been conducted to determine the effects of high doses of vinasse and coal/bagasse ash on soil quality, on the sugar cane plant and on groundwater quality. Analyses of vinasse samples collected at regular intervals of three months during 2005 to 2008 showed that its chemical composition was very variable. However these analyses also showed that as vinasse contained on average 9.37 g/L of K, its fertilization value was mostly as a source of potassium. Nevertheless at an application rate of 100 m3/ha vinasse, it can in addition represent a significant source of N (average of 122 kg N/ha) and of organic matter (average of 8-15 % dry matter). Vinasse also contained heavy metals (Cu, Zn, Ni, Mn, Pb) but their concentrations were negligible and were well below the detection limit of the atomic absorption spectrophotometer (5 mg/kg for Cu, Zn, Ni, Pb and 10mg/kg for Mn). As opposed to vinasse which is acidic in nature (an average pH of 4.5), coal or bagasse ash is alkaline with a pH varying between 7.5 and 11.5 and can therefore be used to amend soil acidity. When applied at 100 T/ha coal ash would represent a significant source of potassium (average of 139 kg K/ha) and of phosphorus (average of 298 kg P/ha) to the sugar cane crop. Due to its low contents of heavy metals (Cu, Zn, Ni, Mn, Pb, and Hg) and to the absence of organic micro pollutants, coal ash is unlikely to enhance the levels of these contaminants in the soil, in the sugar cane crop or in the groundwater. Laboratory studies had demonstrated that coal and bagasse ash were able to strongly adsorb the herbicides atrazine and hexazinone with a sorption coefficient (km) of 0.13 for atrazine and 0.32 for hexazinone. They would thereby act to decrease the movement of these herbicides to groundwater. Analyses of the soils at the study sites showed that the application of vinasse lowered soil pH from an average of 5.9 to 5.4, but this decrease was only temporary and was not significant enough to affect sugar cane growth. In fact the soil pH returned to its initial value shortly after the application of the 100 m3/ha vinasse. On the other hand, coal ash raised soil pH but this rise in pH was likewise not significant to crop growth. Vinasse and coal ash applied at the high rates of 100 m3/ha and 100 T/ha respectively raised the electrical conductivity of the soil, but in spite of this increase, the electrical conductivity remained below the threshold value of 1700 RS/cm recommended for sugar cane. Furthermore if an application rate of 100 m3/ha of vinasse did increase the organic carbon content of certain soils, it was not observed with coal ash at 100 T/ha. Both vinasse and coal ash improved the level of exchangeable calcium and magnesium in the soils. Thus, 12 months after its application at 100 T/ha coal ash raised the level of exchangeable calcium from 3.40 to 6.61 cmol+/kg in the soil at Pamplemousses, whereas during that same period with vinasse at 100 m3/ha, the level of exchangeable calcium rose only from 1.66 to 1.83 cmol+/kg at Union Park, despite the high K content of the vinasse. Field trials laid down in four different agroclimatic zones provided evidence that increasing application rates of vinasse (25, 50 and 100 m3/ha) did not affect cane or sugar yield. On the contrary, results obtained during the three year study period which covered a plant cane crop and two ratoons, had shown that vinasse gave a higher cane yield (an average of 84.9 T/ha/yr for the four sites) than NPK fertilizers alone (an average of 77.3 T/ha/yr for the four sites). As all the treatment plots received the same rate of N and P, this increase in cane yield was in all probability due to a better K nutrition as well as to an improvement in soil organic matter status brought about by the vinasse. On the other hand, coal ash at 100 T/ha definitely impaired cane yield (an average of 67.5 T/ha/yr for the four sites). When it was applied at 50 T/ha, the effect of coal ash on yield was dependent on soil type, being more severe in the highly leached soils of the super humid region at Belle Rive, and less damaging in the soils having undergone less leaching in the humid region of Pamplemousses. Additionally because of their low heavy metal concentrations, vinasse as well as coal ash did not increase the heavy metal contents in the sugar cane plant. To determine the effects of spreading 100 m3/ha of vinasse and 100 T/ha of bagasse ash on groundwater quality, lysimeter studies were conducted at two sites differing in rainfall regimes (3500 mm/yr at Belle Rive and 1500 mm/yr at Réduit). Drainage water percolating at one metre depth was collected after each heavy rainfall event. Analyses of the leachates showed that the vinasse at 100 m3/ha did not enhance loss of N in the form of nitrate. If, as expected, the heavy metals (Cu, Ni and Zn) known to be mobile had been detected in drainage water, their concentrations remained well below the drinking water limits proposed by the World Health Organization (1 mg/L for Cu, 5 mg/L for Zn and 0.02 mg/L for Ni). Fluorescence quenching studies of the dissolved organic matter in vinasse and of its dialysis fractions with copper confirmed the formation of complexes of the dissolved organic matter with copper. As expected, bagasse ash did not increase the levels of heavy metals and organic micropollutants in the drainage water. But bagasse ash as opposed to vinasse moved significantly more nitrate into the groundwater. In fact the limit of 10 mg/L N-NO3- recommended by World Health Organization for drinking water, was exceeded on many occasions during the study period of November 2005 to February 2008 at both Belle Rive and Réduit lysimeters. To conclude, this study has provided some pertinent and very valuable data. Whereas the disposal of high rates of vinasse on agricultural land under sugar cane is acceptable, the same cannot be stated for coal ash which must be avoided as it is would represent a hazard to the sugar cane crop and to groundwater. Indeed, high application rates of coal ash to sugar cane fields in Mauritius will eventually lead to a decrease in sugar cane productivity and to an increase in nitrate contamination of groundwater
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Submitted on : Tuesday, March 16, 2010 - 3:21:13 PM
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Aneeza Soobadar. Impacts agronomiques et environnementaux de l'épandage de vinasse et de cendre de charbon/bagasse sur les terres agricoles de l'Île Maurice. Sciences agricoles. Université d'Avignon, 2009. Français. ⟨NNT : 2009AVIG0621⟩. ⟨tel-00464233⟩



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