Potencial de reuso de sedimento assoreado em açudes na agricultura irrigada na bacia hidrográfica do Banabuiú

Abstract

In semi-arid regions, water availability is endangered in a quantitative and qualitative manner by reservoir sedimentation. Several studies have proposed the reuse of sediment silted in reservoirs as fertilizer, thus transforming nutrient-enriched sediments from waste (in reservoirs) into resource (in agricultural fields). In this work, the potential of reusing sediment as nutrient source for the agricultural sector was assessed in a meso-scale semiarid basin in Brazil, where 1,029 reservoirs were identified. Sedimentation was modelled for the entire reservoir network, accounting for a total sediment deposition of 7 x 105 tons.y-1. The sediment physical and chemical characteristics were estimated based on a survey carried out in four reservoirs, from which a nutrient enrichment was observed in relation to the soils of the basin. The potential of reusing sediment as fertilizer was assessed for maize crops (Zea mays L.) and the sediment mass required to fertilize the soil was computed considering nitrogen as the limiting nutrient, so that the crop requirement would be fully provided by the sediment. To analyse the economic feasibility of the proposed practice, the costs resulting from the application of this technique were compared to those obtained if the area was fertilized by traditional means. In the study area, where reservoirs fall dry frequently and sediments can be removed by regular excavation, soil fertilization with the sediment reuse technique presents lower costs than those observed for application of commercial chemical fertilizers. Savings with soil fertilization with the sediment reuse technique can be as high as 25 %, but costs up to 9 % higher can be obtained if sediment with low nutrient content is used. According to the local conditions and fertilization costs, sediments with nitrogen content above 1.5 g.kg-1 are cost efficient as nitrogen source. However, physicochemical analyses are recommended not only to define the mass of sediment to be used as fertilizer, but also to identify any constraint to the application of the practice. For instance, the sediment of one of the reservoirs in the study area presented a high sodium adsorption ratio, which can contribute to soil salinization and, therefore, is not recommended as nutrient source for the agricultural sector.