SEDIMENT REUSE FROM TROPICAL RESERVOIRS: assessing the suitability of sediment material for soil improvements and impacts of the practice on plant growth

Due to the high rainfall variability in the Brazilian semi-arid region and the occurrence of long periods without rain, society has adopted techniques to cope with drought, with focus on the construction of surface reservoirs. However, silting is causing a decrease in the water storage capacity of those structures, reducing their depth, increasing water losses by evaporation and contributing to the degradation of water quality by adsorbed pollutants. In a context where mitigating solutions are necessary, removal of the nutrient-enriched sediment from the reservoirs’ beds and their subsequent reuse for soil fertilization have been proposed. To assess the potential of the sediment as fertilizer, maize plants were grown under controlled conditions in a greenhouse, considering: i) soil from the region where the sediment was collected with no amendments, ii) soil with 100% of the nitrogen recommendation provided by mineral fertilizer (iii) soil with sediment from São Nicolau reservoir (iii), soil with sediment from São Joaquim reservoir (iv). We observed higher relative chlorophyll content, plant growth and biomass production of maize plants from the soil with added sediment, with a similar behavior to plants growing in the soil with chemical fertilizer. We also found that the silt improves soil structure by increasing the water retention capacity of the soil. We have previously evaluated that this technique is economically feasible and can present savings of up to 30% in relation to traditional fertilization, depending on the characteristics of the sediment. However, sediments from the same hydrographic region may present high spatial variability in their physicochemical characteristics. Therefore, it is relevant to map the spatial distribution of the sediment characteristics. Recently, we demonstrated that diffuse reflectance spectroscopy might be useful to characterize sediments at lower costs and efforts than by laboratory analyses: for instance, regression models for electrical conductivity and clay content performed in the range of good to very good in the study region. A further promising approach is the application of spaceborne imaging spectroscopy to estimate the concentration of elements such as sodium, the electrical conductivity, the content of clay and organic matter in the sediment. The derived information can be used for informed decisions in the application of sediment reuse practice. For example, if the electrical conductivity of the sediment is higher than 4 dS/m, addition of sediment to the soil may prevent plant growth and, therefore, its reuse is not recommended. Thereby, sediment reuse can also potentially promote de-silting of reservoirs, reducing the carbon footprint associated with traditional fertilization and improving the water quality of small reservoirs, the main source of water supply for rural families, by removing nutrients that could return to the water column. In addition, the use of sediments may represent an alternative to increase agricultural production, being less susceptible to market price variation than commercial fertilizers. The CAPES/PROBRAL and the Deutscher Akademischer Austauschdienst (DAAD) are acknowledged for the financial support.